User:Archnapandey/Genetics Practical Protocols
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: You can get rid of ghost boxes by starting to type at the extreme left of each line. In case you need to indent use colon. For bullets use * and # for numbered lists. Pl use bold, titles, sub titles, pics, colors to improve readability. Anyways you could use Open Office.Also you could use this page to list the protocols and put each on a separate sub page as has been done on Acharya_Narendra_Dev_College/Biology_Protocols. Great Work!! savi 16:18, 16 April 2009 (UTC))
AIM: To prove monogenic inheritance using the result of PTC (phenyl thiocarbamide) test, thus confirming the Hardy Weinberg’s Law. REQUIREMENTS: Beaker, PTC, alcohol, strips of Whatmann’s paper, oven. THEORY: This law was proposed by G.H. Hardy and W.Weinberg independently in 1908. They reported that under certain conditions hereditary conservation of genes is the characteristic of population. This is a static condition and the population is non evolving. Such a kind of stability at the genetic level is called genetic equilibrium. Hardy Weinberg Law: the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when- • The population is large enough so that random sampling errors do not affect the allele frequency. • Mating takes place at random. • Mutation does not take place or if it does the rate is same in both directions • All members of population survive and have equal reproductive rates. • There should be no migration among different populations. In reality no population satisfies Hardy Weinberg’s Law. But in large natural populations with little migration and negligible natural selection, the Hardy Weinberg Law may be nearly approximated. Deviations from Hardy Weinberg Law: Mutations: they produce alternate alleles at a given locus and alter the phenotype. In a population, mutation disturbs the genetic equilibrium and gene frequency. Thus gene pool gradually changes with mutant gene appearing with greater frequency. Selection: if a mutation is advantageous then natural selection may preserve it and such characters get inherited and contribute more to the gene pool of next generation. Thus genetic equilibrium is changed. e.g. heterozygous advantage in case of sickle cell anemia. Non random mating: Differential mating is one of the evolutionary forces resulting in abundance of certain genotype at the cost of others. Small population size and genetic drift: the random change in gene frequency occurring by chance and not under the control of natural selection is a genetic drift. In populations genetic drift favors either a loss or fixation of an allele. The rate at which an allele is lost or becomes fixed, depends on the population size.
Significance of the Hardy Weinberg Law: It provides a tool which helps the population geneticists to determine the degree of evolutionary change by comparing allele frequencies at starting point and at some future point. By this law we can calculate the frequencies of homozygous dominant, homozygous recessive and heterozygous carriers in a population.
PROCEDURE: 1. In a beaker, 1% solution of PTC was prepared in 100% alcohol. 2. Several small strips of Whatman’s paper were dipped in at least for 2-3 hours. 3. The strips were allowed to dry in an oven for about half an hour. 4. After drying, the strips were tasted. OBSERVATION:
No. of positive tasters No. of non tasters population size 17 2 19
CALCULATION: According to Hardy Weinberg Law :
p2 + q2 + 2pq = 1
Where, p = allelic frequency of dominant
q = allelic frequency of recessive
2pq = heterozygous dominant frequency
p2 = homozygous dominant frequency
q2 = homozygous recessive frequency
Phenotypic frequency for the population-
Dominant = P2 + 2pq = 17/19 = 0.894
Recessive = q2 = 2/19 = 0.106
Therefore, q = 0.325
And as we know that p+q = 1 ;
P = 1- q = 0.675
Allelic frequency-
Dominant: p = 0.675
Recessive: q = 0.325
Genotypic frequency for population:
Homozygous dominant: p2 = (0.675)2 = 0.45
Homozygous recessive: q2 = (0.325)2 = 0.10
Heterozygous dominant: 2pq = 0.44
RESULT: Monogenetic inheritance in man was proved using result of PTC, thus confirming HARDY-WEINBERG’S LAW
PRECAUTIONS: 1. Whatman’s strips should be dipped in PTC solution for at least 2-3 hours. 2. The strips should always be properly dried before use. 3. Results should be accurately observed. 4. PTC solution was finely prepared
Aim : To calculate linkage number .
Problem 1:
A cross is made between homozygous wild-type female Drosophila (a+a+b+b+ c+c+) and triple-mutant males (aa bb cc) (the order here is arbitrary). Give the constitution of gametes produced by the parents and also genotype of F1 . A test cross of the F1 gave the following phenotypes:
“a+ b c” 18 “a b+ c” 112 “a b c” 308 “a+ b+ c” 66 “a b c+” 59 “a+ b+ c+” 321 “a+ b c+” 102 “a b+ c+” 15
1000
1. Calculate the percentage of crossover between 3 gene pairs 2. Calculate the percentage of double crossover and coefficient of coincidence 3. Make a linkage map of these genes
Problem 2:
A cross was made between C/C , sh/sh , Wx/Wx stocks of maize and one with the genotype c/c , Sh/Sh , wx/wx. Give the constitution of the gametes produced by theparents and the genotype of F1 A test cross of F1 gave the following phenotype :
C sh Wx 2777 c Sh wx 2708 C Sh wx 116 c sh Wx 122 C sh wx 643 c Sh Wx 626 C Sh Wx 4 c sh wx 3 TOTAL 7000
1) Calculate the percentage of crossover between 3 gene pairs 2) Calculate the percentage of double crossover and coefficient of coincidence 3) Make a linkage map of these genes
PENETRANCE
• It is the frequency with which a heritable trait is manifested by individuals carrying the principal gene or genes conditioning it.
• In other words, the percentage of individuals that show at leastr some degree of expression of a mutant genotype defines the penetrance.
• The presence of a gene may not result in a detectable phenotype.
Example:-
The phenotypic expression of many mutant alleles in drosphila is indistinguishable from wild type.If 15 percent of mutant flies show the wild type appearance ,the mutant gene is said to have a penetrance of 85 percent.
VARIABLE EXPRESSIVITY
• Expressivity reflects the range of expression of the mutant genotype. • In other words it is the variation in the degree to which gene is expressed. • the variability with which basic patterns of inheritance are modified, both in degree and in variety, by the effect of a given gene in people of the same genotype.
Example:-
polydactyly may be expressed as extra toes in one generation and extra fingers in another.
AIM : To Study Polyploidy In Allium cepa Root Tip
Things Required : Slide, Colchicine, 1N HCl, Acetocarmine Dye, Onion Root Tip, needle, forceps, Theory : Polyploidy occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploidy is a state different from most organisms which are normally diploid meaning they have only two sets of chromosomes - one set inherited from each parent; polyploidy may occur due to abnormal cell division. It is most commonly found in plants.
Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see Hibiscus Rosa-Sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U. The occurrence of polyploidy is a mechanism of speciation and is known to have resulted in new species of the plant Salsify (also known as "goatsbeard").
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. Polyploidy can be induced in cell culture by some chemicals: the best known is colchicines, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
Polyploid types are labelled according to the number of chromosome sets in the nucleus: • triploid (three sets; 3x), for example the phylum Tardigrada • tetraploid (four sets; 4x), for example Salmonidae fish • pentaploid (five sets; 5x) • hexaploid (six sets; 6x), for example wheat, kiwifruit • oktoploid (eight sets; 8x), for example Acipenser (genus of sturgeon fish) • dekaploid (ten sets; 10x), for example certain strawberries • dodecaploid (twelve sets; 12x), for example the plant Celosia argentea
Autopolyploidy Autopolyploids are polyploids with multiple chromosome sets derived from a single species. Autopolyploids can arise from a spontaneous, naturally occurring genome doubling (for example, the potato). Others might form following fusion of 2n gametes (unreduced gametes). Bananas and apples can be found as triploid autopolyploids. Autopolyploid plants typically display polysomic inheritance, and are therefore often infertile and propagated clonally Allopolyploidy Allopolyploids are polyploids with chromosomes derived from different species. Triticale is an example of an allopolyploid, having six chromosome sets, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid. Some of the best examples of allopolyploids come from the Brassicas, and the Triangle of U describes the relationships among the three common diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploids. Polyploidy in plants Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (eg. potato) and allopolyploids (eg. canola, wheat, cotton) can be found among both wild and domesticated plant species. Most polyploids display heterosis relative to their parental species, and may display novel variation or morphologies that may contribute to the processes of speciation and eco-niche exploitation The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels. Many of these rapid changes may contribute to reproductive isolation and speciation. There are few naturally occurring polyploid conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n=6x=66), although the origin is unclear. Polyploid crops Polyploid plants tend to be larger and better at flourishing in early succession habitats such as farm fields in the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops (and agricultural weeds) may have unintentionally been bred to a higher level of ploidy. The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. In some situations polyploid crops are preferred because they are sterile. For example many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine. Examples of Polyploid Crops • Triploid crops: banana, apple, ginger, watermelon, citrus • Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek • Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit • Octaploid crops: strawberry, dahlia, pansies, sugar cane Polyploidy in animals Examples in animals are more common in the 'lower' forms such as flatworms, leeches, and brine shrimp. Polyploid animals are often sterile, so they often reproduce by parthenogenesis. Polyploid salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death. One of the only known exceptions to this 'rule' is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae). This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of polyploidy in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.. Polyploidy in humans (Aneuploidy) True polyploidy rarely occurs in humans, although it occurs in some tissues (especially in the liver).
Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not," "good," and "fold"). Therefore the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome, whereas polyploidy refers to a numerical change in the whole set of chromosomes.
Polyploidy occurs in humans in the form of triploidy (69,XXX) and tetraploidy (92,XXXX), not to be confused with 47,XXX or 48, XXXX aneuploidy. Triploidy, usually due to polyspermy, occurs in about 2-3% of all human pregnancies and ~15% of miscarriages. The vast majority of triploid conceptions end as miscarriage and those that do survive to term typically die shortly after birth. In some cases survival past birth may occur longer if there is mixoploidy with both a diploid and a triploid cell population present. Triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). Diandry is almost always caused by the fertilization of an egg by two sperm (dispermy). Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry appears to predominate among early miscarriages while digyny predominates among triploidy that survives into the fetal period. However, among early miscarriages, digyny is also more common in those cases <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. In digyny there is typically an asymmetric poorly grown fetus, with marked adrenal hypoplasia and a very small placenta. In diandry, the fetus (when present) is typically normally grown or symmetrically growth restricted, with normal adrenal glands and an abnormally large cystic placenta that is called a partial hydatidiform mole. These parent-of-origin effects reflect the effects of genomic imprinting. Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1-2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism. Mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells. Procedure : 1. Keep the onion root tip in 1% colchicine for 10 -24 hrs at 40 C or for 3-6 hrs at room temperature 2. Transfer in Distilled water 3. Then keep the sample in 1N HCl for 10 minutes 4. wash with water 5. excise the root tip portion 6. add few drops of acetocarmine dye and keep for 15 minutes 7. Tease with needle 8. Heat fix by passing throgh the flame 2 – 3 times 9. Cover with cover slip and tap and view under 100x
Observations: 23 chromosomes observed instead of 16, confirming polyploidy.
Precautions: 1. Slide should not be over heated 2. Stain should not be used in excess 3. Tapping should be done gently.
Aim:
To study polytene chromosome using permanent slides.
Theory:
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881. The hereditary nature of these structures was given by Emil Heitz and Hans Bauer in early 1930s by their studies on Drosophila melanogaster.
To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), forming a giant polytene chromosome. Polyteny is achieved by replication of the DNA several times without nuclear division and the resulting daughter chromatids do not separate and remain aligned side by side. A polytene chromosome in the cells of Drosophila salivary glands has about a thousand DNA molecules arranged side by side, all attached at their centromeres, which arise from ten rounds of DNA replication.
Polytene chromosomes occur in salivary gland, trachea, fat cells and malphigian tubules of many insects. They are known to occur in secretary tissues of other dipteran insects such as the Malpighian tubules of Sciara and also in protists, plants, mammals, or in cells from other insects. Some of the largest polytene chromosomes known occur in larval salivary gland cells of the Chironomid genus Axarus.
In polytene cells the chromosomes are visible during interphase. Polytene chromosomes have characteristic light and dark banding patterns. Dark banding frequently corresponds to inactive chromatin, while light banding is usually found at areas with higher transcriptional activity. The chromomeres (regions in which chromatin is more tightly coiled) alternate with regions where the DNA fibers are loosely folded. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin and general chromatin structure. The banding pattern is specific for each pair of homologous chromosomes. They have their own characteristic morphology and position which permits detailed chromosome mapping. The polytene chromosomes consist of coiled or associated homologous pairs of chromosomes. This sort of association is termed as somatic pairing. This permits the identification of abnormalities like deletion, inversions and duplications as regions looped out of the chromosomes.
In addition to increasing the volume of the cell's nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.
The polytene chromosomes develop swellings at particular points. Such chromosomal swellings are called Chromosome Puffs. They are diffuse uncoiled regions of the polytene chromosome that are sites of active RNA transcription and commonly occur in bands. In polytene chromosomes a series of loops may be given out laterally. These loops are called Balbiani Rings, which are large chromosome puffs. They are rich in DNA and mRNA.
Observation:
1. Polytene chromosomes are giant chromosomes with cable like structure. The maternal and paternal homologues remain associated side by side in somatic pairing which have undergone multiple DNA replications.
2. They show characteristic morphology in which dark bands alternate with interbands.
3. The chromosomes also contained swellings or puffs at particular points (mainly in dark band region). These swelling are called Balbiani Rings.
SEX LINKED INHERITANCE
Y-Linked inheritance ( Hollandric Inheritance) :
• Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. • Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. • The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics. • It is passed from father to all sons i.e. 100% male progeny will be affected. • It does not skip generations.
X-Linked Dominant Inheritance :
• Both males and females are affected ; often more females than males are affected • Does not skip generations. Affected sons must have an affected mother: affected daughters must have either an affected mother or an affected father. • Affected fathers will pass the trait on to all their daughters. • Affected mothers (if heterozygous ) will pass the trait on to ½ of their sons and ½ of their daughters. • An example of an X-linked dominant trait in humans is hypophosphatemia or familial vitamin D-resistant rickets. People with this trait have features that superficially resemble those produced by rickets.
X-Linked Recessive Inheritance:
• More males than females are affected. • Affected sons are usually born to unaffected mothers; thus, the trait skips generations. • Approximately ½ of a carrier (heterozygous) mother’s sons are affected. • It is never passed from father to son. • All daughters of affected fathers are carriers. • An example of an X-linked recessive trait in humans is color blindness.
DOWN’S SYNDROME Down’s syndrome is a form of chromosomal variation or aneuploidy characterized by the presence of an extra chromosome 21 in body. Children with Down’s syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of mental retardation. This varies from person to person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with care and support, most children with down syndrome can grow up to have healthy, happy, productive lives. It is named after Langdon Down, who first described its clinical signs in 1866 and formerly known as “Mongolism”. GENOTYPE Male - Full/partial (mosaic) Female -Disorder was identified as chromosome 21 trisomy by Jerome Lejuene in 1959. INCIDENCE RATE The incidence of Down syndrome is estimated at one per 800 to one per 1000 births. The incidence rate increases with increase in maternal age. CAUSES • Maternal age: influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24 , the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. About 75% of these maternal non –disjunctions occur during meiosis I, with the reminder occurring during meiosis II. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of down syndrome manifesting in pregnancies in older mothers. • NON-DISJUNCTION: Non-Disjunction event during gametogenesis or early cell division during embryogenesis may result in full or mosaic Down’s syndrome respectively. Out of 95% of total cases, 88% causes result due to non-disjunction event during female gametogenesis and 8% during gametogenesis in males.
• MOSAICISM: Trisomy 21 is usually caused by non-disjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic. Down syndrome (46, XX/47, XX, +21). This can occur in one of two ways
1. A non-disjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21.
2. Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the causes of 1-2% of the observed Down syndrome
SYMPTOMS:
Most children with Down syndrome have:
• Distinct facial features, such as a flat face, small ears, flat nasal bridge, slanting eyes, oblique parperal fiscers and a small mouth with protruding tongue. • A short neck and short arms and legs. • Excessive space between large toe and second toe. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below average intelligence. • Occipital is flat. • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is highly consistent feature that is helpful in making a diagnosis.
Many children with Down syndrome are also born with heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Luckily, most of these problems can be treated
PATAU’S SYNDROME
Patau’s syndrome, also called trisomy 13, is a congenital disorder associated with the presence of an extra copy of chromosome 13. The extra chromosome 13 causes numerous physical and mental abnormalities, especially heart defects. Patau syndrome is named after Dr. Klaus Patau, who reported the syndrome and its association with trisomy in 1960.
Genetics:
75 to 80% of the cases of Patau syndrome are caused by trisomy of chromosome 13. Some of these cases are the result of a total trisomy, while others are the result of a partial trisomy. Partial trisomy generally causes less severe physical symptoms than full trisomy. 10% of these cases are of the mosaic type, in which only some of the body’s cells have the extra chromosome. The physical symptoms of the mosaic form of Patau syndrome depends on the number and type of cells that carry the trisomy. Most of the cases of trisomy are not passed on from one generation to the next. Usually they result from a malfunction in the cell division (mitosis) that occurs after conception. At least 75% of the cases of Patau syndrome are caused by errors in chromosome replication that occur after conception. The remaining 25% are caused by the inheritance of translocations of chromosome 13 with other chromosomes within the parental chromosomes. In these cases, a portion of another chromosome switches places with a portion of chromosome 13. This leads to errors in the genes on both chromosome 13 and the chromosome from which the translocated portion originated.
Karyotype:
Full trisomy: 47,XX,+13
47,XY,+13
Mosaic trisomy: 47,XX/46,XX,+13
Incidence Rate:
Incidence rate is 1 in 6000 births, births may be falling due to prenatal screening and selective termination of pregnancy. The risk of Patau syndrome seems to increase with the mother’s age, particularly if she is over 30 when pregnant. Male and female children are equally affected, and the syndrome occurs in all races.
Signs and symptoms:
• Mental and motor retardation. • Polidactyly (extra digits) • Microcephaly • Low set ears • Holoprospncephaly (failure of the fore brain two divide properly) • Heart defects (80% of the cases) • Structural eye defects, including Micropthalmia, retinal Dysplasia or retinal detachment, cortical visual loss and optic nerve hypoplasia • Cleft palate or hare lip (not complete fusion of soft and hard palate) • Meningomyelocete (spinal defect or motor abnormality) • Omphalocete (abdominal defect) • Abnormal Genitalia • Abnormal Palm pattern • Overlapping of fingers over thumb • Micrognathia (small jaws) • Hypertonia (low muscle tone) • Cryptorchoidism
The life expectancy is very limited. Most die in the first week and few survive beyond a year. The median age of death is 2.5 days. More than 80% die within a month. Only 5 % last 6 months. The maximum life Span is 21 years. In many cases, spontaneous abortion (miscarriage) occurs and the fetus does not survive to term. In other cases, the affected individual is stillborn.
PHILADELPHIA CHROMOSOME
Philadelphia chromosome is referred to an abnormally short chromosome 22, one of the two chromosomes involved in translocation with chromosome 9 in this condition. This chromosome abnormality causes chronic myeloid leukemia (CML) and is also seen in some cases of acute lymphoblastic leukemia (AML). The Philadelphia chromosome was first discovered and described in 1960 by Peter Nowell from University of Pennsylvania School of medicine and David Hungerford from the Fox Chase Cancer Center’s Institute for cancer research and was therefore named after the city in which both centers were located. The mechanism by which the Philadelphia chromosome arises as a result of translocation was however identified by Janet D. Rowley at the University of Chicago in 1973.
Molecular Genetics:
Due to translocation, a part of bcr (Breakpoint Cluster Region) gene from chromosome 22 gets fused with part of the abl (Abelson) gene on chromosome 9. The abl gene (22, q11) encodes for tyrosine kinase protein in a regulated manner. During reciprocal translocation, parts of these two chromosomes swap pieces, which lead to fusion of abl-bcr on shorter chromosome 22 (Ph chromosome). This fused gene encodes for a 21 kD protein with tyrosine kinase enzyme activity but in uncontrolled amount as regulatory sequences may have been dissociated from gene during translocation. The BCR-ABL protein is expressed at unregulated levels and interacts with interleukin β3 receptor subunit, which in turn activates number of cell cycle controlling proteins thus speeding the cell division. It also results in inhibition of DNA repair, causing genomic instability. The efficacy in CML of a drug that inhibits the BCR-ABL tyrosine kinase has provided the final proof that the BCR-ABL oncoprotein is the unique cause of CML. Nomenclature:
t(9;22)q34;q11) based on International System for Human Cytogenetic Nomenclature.
Incidence Rate:
95% of patients suffering from Chronic Myeloid Leukemia while only 10-12% patients suffering from acute lymphoblast leukemia show this abnormality.
Disease and Symptoms:
Philadelphia chromosome causes Chronic Myeloid Leukemia (CML). This translocation occurs in single bone marrow cell and through process of clonal expansion, gives rise to leukemia. A progressive blood and bone marrow disease usually occurs during or after middle age and rarely in children. Normally the bone marrow makes blood stem cells that develop into mature blood cells after some time. A blood stem cell may become a myeloid stem cell or a lymphoid cell that develops into white blood cell (WBC). In CML, too many blood stem cells develop into WBCs. These WBCs are abnormal and do not develop into healthy WBC’s. These cells build up in bone marrow leaving less room for healthy cells resulting in anemia.
Symptoms:
• Mild to moderate anaemia • Fatigue • Low grade fevers or sweats • Fullness in the abdomen caused by enlarged spleen • left-upper-quadrant pain caused by splenic infarction
Aim: To verify Mendelian law and study deviations from laws.
Observation: The observed ratio of different variety of seeds was 22:7.
Result: Since the observed ratio is close to the phenotypic ratio observed in case of a monohybrid cross, we hypothesize that the above case represents monohybrid cross.
Theory: A Monohybrid cross is a cross between parents who are heterozygous at one locus. It is a breeding experiment dealing with a single character. For example, a monohybrid cross between two pure-breeding plants (homozygous for their respective traits), one with yellow seeds (the dominant trait) and one with green seeds (the recessive trait), would be expected to produce an F1 (first) generation with only yellow seeds because the allele for yellow seeds is dominant to that of green. A monohybrid cross compares only one trait. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%. Maternal B b Paternal B BB Bb b Bb bb It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact. For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross. Discussion: On calculating the chi-square value it was found to be 0.011, when this value was compared with the standard table showing relation between degree of freedom and chi square value for 5% probability it was found to be less. Thus the hypothesis is accepted.
KLINEFELTER’S SYNDROME
Klinefelter’s syndrome is a genetic disorder, which affects males, causing reduced fertility and development of small testicles. Affected individuals have atleast one Y chromosome and atleast two X chromosomes. It is named after Dr. Harry klinefelter , an endocrinologist who first described it in 1942.
Genotype:
It is a condition caused by chromosome nondisjunction in males; affected individuals have a pair of X sex chromosomes instead of just one. About 50-60% of the cases are due to maternal nondisjunction (75% meiosis I errors). Despite the relatively mild phenotypic features of this disorder, it is estimated that at least half of the 47, XXY conceptions are spontaneously aborted. In mammals with more than one X chromosome, the genes on all but one X chromosome are not expressed; this is known as X inactivation. This happens in XXY males as well as normal XX females. A few genes located in the pseudoautosomal regions, however, have corresponding genes on the Y chromosome and are capable of being expressed. These triploid genes in XXY males may be responsible for symptoms associated with Klinefelter's syndrome.
Karyotype: 47,XXY Males
48, XXXY Males
Incidence rate:
The condition is reported in roughly 1 out of every 500 males.
Signs and Symptoms:
• Taller than average individuals with longer arms and legs • Slightly feminized physique with reduced body hair • Small testis (microorchidism). • Serum testosterone levels are lower than normal. • Urinary gonadotropin levels are elevated. • Infertility (azoospermia) may result from atrophy of seminiferous tubules. • Gynecomastia (breast development). • Mild degree of mental retardation. • Language learning impairment but not mentally retarded.
Aim: To study the human karyotype.
Theory:
A karyotype is the characteristic chromosome complement of a eukaryote species.The preparation and study of karyotypes is part of cytogenetics.
In normal diploid organisms, autosomal chromosomes are present in two identical copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is also known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.The chromosomal pairs are arranged in descending order of size with autosomes followed by sex chromosomes.
The study of karyotypes is made possible by staining: usually a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn foetus can be determined by observation of interphase cells .
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for women contain two X chromosomes and are denoted 46,XX; men have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Purpose of Karyotype:
Karyotypes can be used for many purposes:
- to study chromosomal aberrations
- to study cellular function
- to study taxonomic relationships, or
- to gather information about past evolutionary events.
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well suited example is the Philadelphia chromosome, a translocation mutation associated with chronic myelogenous leukemia and less often wuth acute lymphoblastic leukemia.
Observations in a Karyotype:
Six different characteristics of karyotypes are usually observed and compared:
- differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication.
- differences in the position of centromeres. This is brought about by translocations.
- differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
- differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.
- differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
- differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
After sorting by size, chromosomes are further classified as :
Position of the centromere:
Metacentric- having centromere in the centre.
Submetacentric- having centromere near the centre.
Acrocentric- having centromere near the end.
Telocentric- having centromere at the end.
Number of centromeres:
Monocentric- having a single centromere.
Dicentric-having two centromeres.
Polycentric-having many centromeres.
Holocentric-absence of functional centromere.Generally, a dispersed centromere is present over the entire chromosome.
Variation is often found:
- between the two sexes
- between the germ-line and soma (between gametes and the rest of the body)
- between members of a population (chromosome polymorphism)
- geographical variation between races
- mosaics or otherwise abnormal individuals
Human Chromosome Groups:
Group Chromosomes Description A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric B 4,5 Large; submetacentric with two arms very different in size C 6–12,X Medium size; submetacentric D 13–15 Medium size; acrocentric with satellites E 16–18 Small; 16 is metacentric but 17 and 18 are submetacentric F 19,20 Small; metacentric G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the Y Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller than chromosome 22.
Depiction Of Karyotypes: Cytogenetics employs several techniques to visualize different aspects of chromosomes. G-banding :It is obtained with Giemsa stain following digestion of chromosomes with trypsin.It brings out sulfur rich proteins. It yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300-400 bands in a normal, human genome. R-banding :It is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).It requires heat treatment so as to denature the bonds between adenine and thymine (and other regions get stained). C-banding: Giemsa binds to constitutive heterochromatin, so it stains centromeres. Q-banding: It is a fluorescent pattern obtained using quinacrine mustard for staining. The pattern of bands observed using fluorescent microscope is very similar to that seen in G-banding. T-banding: visualize telomeres. In the "classic" karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2) Chromosome abnormalities: Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells. Chromosomal abnormalities that lead to disease in humans include Turner syndrome, Klinefelter syndrome, Edwards syndrome, Down syndrome, Patau syndrome, trisomy 8, trisomy 9 and trisomy 16. Some disorders arise from loss of just a piece of one chromosome, including Cri du chat, 1p36 Deletion syndrome, Angelman syndrome. All such abnormalities can be diagnosed using a karyotype.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: Given ratio of seeds 4 Grams colored seeds 8 Black colored seeds 4 Green colored seeds Result:
Since the observed ratio is close to the phenotypic ratio observed in case of incomplete dominance, we hypothesize that the above case represents incomplete dominance. This case represents a deviation from Mendelian laws.
Theory:
Mendel gave the law of inheritance for Monohybrid as well as Dihybrid cross for F1 & F2 Generation. For a monohybrid cross between RR tall character (dominant) and rr dwarf character (recessive), the mendelian ratio of phenotype for F2 Generation is 3:1 (3 tall, 1 dwarf) and genotypic ratio is 1:2:1.
The incomplete dominance was observed in Antirrhinum majus. The RR-red character, rr-white when crossed the flowers in F1 generation were all pink. When F1 generation flower is self crossed the flowers of F2 generation were 1(red), 2 (pink), 1 (white) which shows that 1:2:1 was followed.
RR rr
F1 generation Rr (all pink)
F2 generation R r
RR Rr
Rr Rr
R
r
RR-red Rr-pink rr-white
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance
Expected ratio: 1:2:1 Observed ratio: 1/4*8 =2 1/2*8=4 1/4*8=2 Degree of freedom for given example (n-1) (3-1)=2 X2 test is applied. X2= (obs-exp)2 /exp + (obs-exp)2 /exp +(obs-exp)2 /exp X2=(4-4)2 /4+ (8-8)2 /8+(4-4)2 /4=0
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis.
EDWARD SYNDROME
The Edward's syndrome, which got its name after the famous doctor, Dr. John Edward (1960), is a genetic chromosomal disorder caused by an error in cell division resulting in an additional third chromosome 18. Edward's syndrome is manifested by a characteristic pattern of anatomical defects in a newborn child and has major implications for its health and survival.
Genetics:
Edward’s syndrome is characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or part (such as due to translocations). In majority of Edward's syndrome cases, all cells of the individual contain additional chromosome 18. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. With a translocation, the person has a partial trisomy for chromosome 18 and the abnormalities are often less than for the typical Edward’s. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edward’s syndrome.
Karyotype: 47,XY,+18
47,XX,+18
46,XX/47,XX,+18 (Mosaic condition)
Incidence Rate: Edward's syndrome is second most common after Down syndrome, occurs in approximately one among 3000 to 6000 births. The incidence rate increases as the mother's age increases. Signs and Symptoms: • Growth Deficiency, • Abnormal skull shape and facial features, • Clenched hands, • Rocker bottom feet, • Cardiac and renal abnormalities, • Horse shoe-shaped kidney, • Low set and deformed ears, • Prominent external genitalia, • Small placenta, • Mental retardation, • Hypotonia, • Microcephaly, • Micronagthia (abnormally small jaw), • Cleft lip, • Respiratory Failure -Apnea The survival rate of Edward’s Syndrome is very low, resulting from heart abnormalities, kidney malformations, and other internal organ disorders. About 95% die in utero. Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life. Major causes of death include apnea and heart abnormalities.
Aim: To verify Mendelian laws and study deviations from laws.
Observation: The observed ratio was 30:2.
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of duplicate genes, we hypothesize that the above case represents duplicate genes.
Theory: Gene duplication (or chromosomal duplication) is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination or duplication of an entire chromosome. The second copy of the gene is often free from mutations . So it will have no deleterious effects to its host organism. The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a different function and/or structure. Gene duplication plays a major role in evolution. Plants are the most prolific genome duplicators. For example, wheat is hexaploid (a kind of polyploid), meaning that it has six copies of its genome. There can be complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene.
e.g. Petal color in snapdragon plant.
P: AABB aabb
(Red colour) (white colour)
F1: AaBb
AaBb AaBb
One allele is sufficient to produce the pigment. Whenever a dominant gene is present, the trait is expressed. The phenotypic ratio observed in F2 generation in this case is 15:1.
Discussion: Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Since, the chi square vale is zero, we accept the abovementioned hypothesis. CRI DU CHAT SYNDROME
Cri du chat syndrome or Lejeune’s syndrome, was first described by Jérôme Lejeune in 1963. It is also called as 5p minus or 5p deletion syndrome. It is a group of symptoms that result from missing a piece of chromosome number 5. The syndrome’s name is based on the infant’s cry, which is high-pitched and sounds like a cat.
Genetics:
Cri du chat syndrome (CdCS) is due to a partial deletion of the short arm of chromosome number 5. Approximately 80% of cases results from a sporadic de novo deletion, while about 10-15% are due to unequal segregation of a parental balanced translocation, where the 5p monosomy is often accompanied by a trisomic portion of the genome. The phenotypes in these individuals may be more severe than in those with isolated monosomy of 5p because of this additional trisomic portion of the genome. Most cases involve terminal deletions with 30-60% loss of 5p material. Fewer than 10% of cases have other rare cytogenetic aberrations (eg, interstitial deletions, mosaicisms, rings and de novo translocations). The deleted chromosome 5 is paternal in origin in about 80% of the cases.
Karyotype: 46, XY, del [5p region]
46, XX, del [5p region]
Incidence Rate: 1 in 20,000 to 50,000 live births
Signs and symptoms:
➢ feeding problems because of difficulty swallowing and sucking ➢ low birth weight and poor growth ➢ severe cognitive, speech, and motor delays ➢ widely set eyes ➢ partial webbing or fusing of fingers or toes ➢ behavioral problems such as hyperactivity, aggression, tantrums ➢ unusual facial features which may change over time ➢ excessive dribbling ➢ constipation ➢ microcephaly(small head) ➢ growth retardation ➢ a round face with full cheeks ➢ low birth weight ➢ hypotonia ➢ epicanthal folds (folds of skin above eyes) ➢ down-slanting palpebral fissures ➢ flat nasal bridge ➢ down-turned mouth and high palate ➢ micrognathia(small jaw) ➢ low-set ears ➢ short fingers ➢ single palmar creases(simian crease) ➢ and cardiac defects(ventricular and atrial septal defects)
The Cri du chat affected people are fertile and can reproduce.
Less frequently encountered findings include:
➢ cleft lip and palate ➢ gut malrotation ➢ inguinal hernia ➢ dislocated hips ➢ cryptorchidism(undescended testis) ➢ rare renal malformations (eg horseshoe kidneys)
Late childhood and adolescence findings include:
➢ severe mental retardation ➢ microcephaly ➢ coarsening of facial features ➢ prominent supraorbital ridges ➢ deep-set eyes ➢ single line on the palm of hand (simian crease) ➢ Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time ➢ In males, testes are often small, but spermatogenesis is thought to be normal.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: 9 red seeds; 6 brown seeds
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of complementary genes, we hypothesize that the above case represents complementary genes. This case represents a deviation from Mendelian laws. Theory: Complementary Genes are those nonallelic genes, which independently show a similar effect but produce a new trait when present together in dominant form. Complementary genes were first studied by Bateson and Punnet (1906) in case of flower color of Sweet Pea (Lathyrus odoratus). Here, the flower color is purple if dominant alleles of two genes are present together (C-P-). The color is white if the double dominant condition is absent (ccP-, c-PP, ccpp ). If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure line, homozygous recessive plant with white flowers, the F1 plant will have colored flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but epistatic interactions of the C and P genes will give a modified 9:7 ratio. The following table describes the interactions for each genotype and how the ratio occurs. Genotype Flower Color Enzyme Activities 9 C_P_ Flowers colored; anthocyanin produced Functional enzymes from both genes 3 C_pp Flowers white; no anthocyanin produced p enzyme non-functional 3 ccP_ Flowers white; no anthocyanin produced c enzyme non-functional 1 ccpp Flowers white; no anthocyanin produced c and p enzymes non-functional
It is believed that the dominant gene C produces an enzyme which converts the raw material into chromatogen. The dominant gene P gives rise to an oxidase enzyme that changes chromatogen into purple anthocyanin pigment. This is confirmed by mixing the extract of the two types of flowerts when purple color is formed. Thus purple color formation is two step reaction and the two genes cooperate to form the ultimate product.
Raw material A Chromagen Anthocyanin
Gene C enz Gene P enz
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Calculated chi square value is 0.07. Degrees of freedom =n-1= 2-1 =1 Chi square value from the table is 3.84
0.07<3.84
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis. DOWN’S SYNDROME
Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is the best known and most common chromosome-related disease syndrome formerly known as “mongolism”. It is named after Langdon Down, who first described its clinical signs in 1866.
Genetics:
About 95% of the cases are caused by non-disjunction, with most of the remainder being caused by chromosome translocation. The extra chromosome is contributed by the mother in 90-95% cases. About 75% of these maternal non-disjunctions occur during meiosis I, with the remainder occurring during meiosis II. There is a strong correlation between maternal age and the risk of producing a child with Down’s syndrome. Some patients with Down’s syndrome have a total of 46 chromosomes instead of 47, but in such cases a translocation has joined a part of long arm of chromosome 21 with the long arm of chromosome 14 (14q21q). Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic Down syndrome. This can occur in one of two ways: • a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21. • a Down’s syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
Karyotype: 47,XY,+21
47,XX,+21
46,XX/47,XX,+21 (Mosaic condition)
Incidence Rate: Down’s syndrome is the most common autosomal aneuploidy seen among live births. It is seen in approximately 1/700 live birth, making it the most common aneuploidy condition compatible with survival to term. The risk for mothers less than 25 years of age to have the trisomy is about 1/1500; at 40 years of age, 1/1000; at 45, 1/400. Pregnant women over 45 are a special high-risk group.
Signs and Symptoms:
• Distinct facial features, such as a flat face, small ears, slanting eyes, and a small mouth. • A short neck and short arms and legs. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below-average intelligence. • Occipital is flat • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is a highly consistent feature that is helpful in making a diagnosis. • Fertility amongst both males and females is reduced. Approx 75% of trisomy 21 conceptions are spontaneously aborted, the Down’s syndrome female’s risk of producing affected live-born offspring is considerably lower than 50%. • Heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Most of these problems can be treated.
XYY SYNDROME
XYY syndrome or Jacob's syndrome is a rare chromosomal disorder that affects males. It is caused by an aneuploidy (trisomy) of the sex chromosomes, Y chromosome, thus a human male receives an extra Y chromosome in each cell. The first published report of a man with a 47,XYY Karyotype was by Avery A. Sandberg and colleagues at Roswell Park Memorial Institute in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence that was karyotyped because he had a daughter with Down syndrome.
Genetics:
Males normally have one X and one Y chromosome. However, individuals with Jacob's syndrome have one X and two Y chromosome. Males with Jacob's syndrome, also called XYY males. 47, XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in chromosome separation during metaphase I or metaphase II called no disjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells.
In some cases, the addition of an extra Y chromosome results from non disjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics
Karyotype: 47, XYY.
46, XY/47, XYY mosaics
Incidence Rate: About 1 in 1,000 boys are born with a 47,XYY Karyotype. The incidence of 47,XYY is not affected by advanced paternal or maternal age. Signs and Symptoms: Physical traits: Most often, the extra Y chromosome causes no unusual physical features or medical problems. • 47, XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height. • Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47, XYY. Testosterone levels (prenatally and postnatally) are normal in 47, XYY males. Most 47, XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason. Behavioral traits: • 47, XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. • As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings • Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47, XYY and are managed no differently than in 46,XY males. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
TRIPLE X SYNDROME
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The first published report of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland, in 1959.
Genetics: The condition is also known as triplo-X, trisomy X, XXX syndrome, and 47,XXX aneuploidy. Unlike other chromosonal conditions (such as fragile X), there is usually no distinguishable difference between women with triple X and the rest of the female population. Triple X syndrome is usually not inherited, but occurs as a random event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the nondisjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of her cells. In some cases, trisomy X occurs during cell division in early embryonic development. The additional X chromosome can come from either the maternal or paternal side. The condition is verified only by karyotype testing as it may not be distinguishable phenotypically. • Full trisomy: In this case, trisomy X occurs during gamete formation. • Mosaic trisomy: Some females with triple X syndrome have an extra X chromosome in only some of their cells. In this case, trisomy X occurs during embryogenesis.
Karyotype:
• Full trisomy: 47,XXX • Mosaic: 46,XX/47,XXX Incidence Rate: Triple X syndrome occurs in around 1 in 1,000 live female births. Signs and symptoms: • Due to inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell and two Barr bodies are visible in somatic cell . Thus, triple X syndrome most often causes no unusual physical features or medical problems. • Females with the condition may have menstrual irregularities. • Although they rarely exhibit severe mental impairments, they have an increased risk of learning disabilities, delayed speech, and language skills. • a lanky/youthful appearance with increased facial beauty has been described, or in some instances varying degrees of androgeny, but these cases usually reflect traits present in near relatives. • Most women with triple X have normal sexual development and are able to conceive children. • A few may experience an early onset of menstruation. • Early menopause. • Triple X women are rarely diagnosed, apart from pre-natal testing methods, such as amniocentesis. Most medical professionals do not regard the condition a disability. However, such status can be sought by parents for early intervention treatment if mild delays are present.
TURNER’S SYNDROME
The syndrome is named after Henry Turner, an Oklahoma endocrinologist, who described it in 1938. The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London. It was found in a 14-year-old girl with signs of Turner syndrome.
Genetics: Turner’s syndrome encompasses several conditions, of which monosomy XO is the most common. Instead of the normal XX sex chromosomes for a female, (or XY for a normal male) only one X chromosome is present and fully functional; in rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). In Turner’ssyndrome, female sexual characteristics are present but generally underdeveloped. The risk factors for Turner’s syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome. There is currently no known cause for Turner syndrome, though there are several theories surrounding the subject. Karyotype: • Full Monosomy: 45,XO • Mosaic Monosomy: 46,XX/45,XO Incidence Rate: Approximately 98% of all fetuses with Turner syndrome result in miscarriage. Turner syndrome accounts for about 10% of the total number of spontaneous abortions in the United States. The incidence of Turner syndrome in live female births is believed to be 1 in 2500. Signs and Symptoms: -Short stature -Lymphedema (swelling) of the hands and feet -Broad chest , poor breast development and widely-spaced nipples -Low hairline -Low-set ears, hearing loss -Reproductive sterility, rudimentary ovaries, gonadal streak (underdeveloped gonadal structures) -Amenorrhea or the absence of a menstrual period -Increased weight, obesity -Shortened metacarpal IV (of hand) -Characteristic facial features,visual impairments -Webbing of the neck (webbed neck) -Congenital heart disease-Coarctation of the aorta -Horseshoe kidney -Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. -Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. -Approximately one-third of all women with Turner syndrome have a thyroid disorder. -Moderately increased risk of developing diabetes. -Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. -Women with Turner syndrome are almost universally infertile. Even when pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner’s Syndrome or Down’s Syndrome. -Other symptoms may include a small lower jaw (micrognathia), cubitus valgus (turned-out elbows), soft upturned nails, palmar crease and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition, and no two individuals will share the same symptoms.
AIM: To prove monogenic inheritance using the result of PTC (phenyl thiocarbamide) test, thus confirming the Hardy Weinberg’s Law. REQUIREMENTS: Beaker, PTC, alcohol, strips of Whatmann’s paper, oven. THEORY: This law was proposed by G.H. Hardy and W.Weinberg independently in 1908. They reported that under certain conditions hereditary conservation of genes is the characteristic of population. This is a static condition and the population is non evolving. Such a kind of stability at the genetic level is called genetic equilibrium. Hardy Weinberg Law: the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when- • The population is large enough so that random sampling errors do not affect the allele frequency. • Mating takes place at random. • Mutation does not take place or if it does the rate is same in both directions • All members of population survive and have equal reproductive rates. • There should be no migration among different populations. In reality no population satisfies Hardy Weinberg’s Law. But in large natural populations with little migration and negligible natural selection, the Hardy Weinberg Law may be nearly approximated. Deviations from Hardy Weinberg Law: Mutations: they produce alternate alleles at a given locus and alter the phenotype. In a population, mutation disturbs the genetic equilibrium and gene frequency. Thus gene pool gradually changes with mutant gene appearing with greater frequency. Selection: if a mutation is advantageous then natural selection may preserve it and such characters get inherited and contribute more to the gene pool of next generation. Thus genetic equilibrium is changed. e.g. heterozygous advantage in case of sickle cell anemia. Non random mating: Differential mating is one of the evolutionary forces resulting in abundance of certain genotype at the cost of others. Small population size and genetic drift: the random change in gene frequency occurring by chance and not under the control of natural selection is a genetic drift. In populations genetic drift favors either a loss or fixation of an allele. The rate at which an allele is lost or becomes fixed, depends on the population size.
Significance of the Hardy Weinberg Law: It provides a tool which helps the population geneticists to determine the degree of evolutionary change by comparing allele frequencies at starting point and at some future point. By this law we can calculate the frequencies of homozygous dominant, homozygous recessive and heterozygous carriers in a population.
PROCEDURE: 1. In a beaker, 1% solution of PTC was prepared in 100% alcohol. 2. Several small strips of Whatman’s paper were dipped in at least for 2-3 hours. 3. The strips were allowed to dry in an oven for about half an hour. 4. After drying, the strips were tasted. OBSERVATION:
No. of positive tasters No. of non tasters population size 17 2 19
CALCULATION: According to Hardy Weinberg Law :
p2 + q2 + 2pq = 1
Where, p = allelic frequency of dominant
q = allelic frequency of recessive
2pq = heterozygous dominant frequency
p2 = homozygous dominant frequency
q2 = homozygous recessive frequency
Phenotypic frequency for the population-
Dominant = P2 + 2pq = 17/19 = 0.894
Recessive = q2 = 2/19 = 0.106
Therefore, q = 0.325
And as we know that p+q = 1 ;
P = 1- q = 0.675
Allelic frequency-
Dominant: p = 0.675
Recessive: q = 0.325
Genotypic frequency for population:
Homozygous dominant: p2 = (0.675)2 = 0.45
Homozygous recessive: q2 = (0.325)2 = 0.10
Heterozygous dominant: 2pq = 0.44
RESULT: Monogenetic inheritance in man was proved using result of PTC, thus confirming HARDY-WEINBERG’S LAW
PRECAUTIONS: 1. Whatman’s strips should be dipped in PTC solution for at least 2-3 hours. 2. The strips should always be properly dried before use. 3. Results should be accurately observed. 4. PTC solution was finely prepared
Aim : To calculate linkage number .
Problem 1:
A cross is made between homozygous wild-type female Drosophila (a+a+b+b+ c+c+) and triple-mutant males (aa bb cc) (the order here is arbitrary). Give the constitution of gametes produced by the parents and also genotype of F1 . A test cross of the F1 gave the following phenotypes:
“a+ b c” 18 “a b+ c” 112 “a b c” 308 “a+ b+ c” 66 “a b c+” 59 “a+ b+ c+” 321 “a+ b c+” 102 “a b+ c+” 15
1000
1. Calculate the percentage of crossover between 3 gene pairs 2. Calculate the percentage of double crossover and coefficient of coincidence 3. Make a linkage map of these genes
Problem 2:
A cross was made between C/C , sh/sh , Wx/Wx stocks of maize and one with the genotype c/c , Sh/Sh , wx/wx. Give the constitution of the gametes produced by theparents and the genotype of F1 A test cross of F1 gave the following phenotype :
C sh Wx 2777 c Sh wx 2708 C Sh wx 116 c sh Wx 122 C sh wx 643 c Sh Wx 626 C Sh Wx 4 c sh wx 3 TOTAL 7000
1) Calculate the percentage of crossover between 3 gene pairs 2) Calculate the percentage of double crossover and coefficient of coincidence 3) Make a linkage map of these genes
PENETRANCE
• It is the frequency with which a heritable trait is manifested by individuals carrying the principal gene or genes conditioning it.
• In other words, the percentage of individuals that show at leastr some degree of expression of a mutant genotype defines the penetrance.
• The presence of a gene may not result in a detectable phenotype.
Example:-
The phenotypic expression of many mutant alleles in drosphila is indistinguishable from wild type.If 15 percent of mutant flies show the wild type appearance ,the mutant gene is said to have a penetrance of 85 percent.
VARIABLE EXPRESSIVITY
• Expressivity reflects the range of expression of the mutant genotype. • In other words it is the variation in the degree to which gene is expressed. • the variability with which basic patterns of inheritance are modified, both in degree and in variety, by the effect of a given gene in people of the same genotype.
Example:-
polydactyly may be expressed as extra toes in one generation and extra fingers in another.
AIM : To Study Polyploidy In Allium cepa Root Tip
Things Required : Slide, Colchicine, 1N HCl, Acetocarmine Dye, Onion Root Tip, needle, forceps, Theory : Polyploidy occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploidy is a state different from most organisms which are normally diploid meaning they have only two sets of chromosomes - one set inherited from each parent; polyploidy may occur due to abnormal cell division. It is most commonly found in plants.
Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see Hibiscus Rosa-Sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U. The occurrence of polyploidy is a mechanism of speciation and is known to have resulted in new species of the plant Salsify (also known as "goatsbeard").
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. Polyploidy can be induced in cell culture by some chemicals: the best known is colchicines, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
Polyploid types are labelled according to the number of chromosome sets in the nucleus: • triploid (three sets; 3x), for example the phylum Tardigrada • tetraploid (four sets; 4x), for example Salmonidae fish • pentaploid (five sets; 5x) • hexaploid (six sets; 6x), for example wheat, kiwifruit • oktoploid (eight sets; 8x), for example Acipenser (genus of sturgeon fish) • dekaploid (ten sets; 10x), for example certain strawberries • dodecaploid (twelve sets; 12x), for example the plant Celosia argentea
Autopolyploidy Autopolyploids are polyploids with multiple chromosome sets derived from a single species. Autopolyploids can arise from a spontaneous, naturally occurring genome doubling (for example, the potato). Others might form following fusion of 2n gametes (unreduced gametes). Bananas and apples can be found as triploid autopolyploids. Autopolyploid plants typically display polysomic inheritance, and are therefore often infertile and propagated clonally Allopolyploidy Allopolyploids are polyploids with chromosomes derived from different species. Triticale is an example of an allopolyploid, having six chromosome sets, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid. Some of the best examples of allopolyploids come from the Brassicas, and the Triangle of U describes the relationships among the three common diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploids. Polyploidy in plants Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (eg. potato) and allopolyploids (eg. canola, wheat, cotton) can be found among both wild and domesticated plant species. Most polyploids display heterosis relative to their parental species, and may display novel variation or morphologies that may contribute to the processes of speciation and eco-niche exploitation The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels. Many of these rapid changes may contribute to reproductive isolation and speciation. There are few naturally occurring polyploid conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n=6x=66), although the origin is unclear. Polyploid crops Polyploid plants tend to be larger and better at flourishing in early succession habitats such as farm fields in the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops (and agricultural weeds) may have unintentionally been bred to a higher level of ploidy. The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. In some situations polyploid crops are preferred because they are sterile. For example many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine. Examples of Polyploid Crops • Triploid crops: banana, apple, ginger, watermelon, citrus • Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek • Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit • Octaploid crops: strawberry, dahlia, pansies, sugar cane Polyploidy in animals Examples in animals are more common in the 'lower' forms such as flatworms, leeches, and brine shrimp. Polyploid animals are often sterile, so they often reproduce by parthenogenesis. Polyploid salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death. One of the only known exceptions to this 'rule' is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae). This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of polyploidy in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.. Polyploidy in humans (Aneuploidy) True polyploidy rarely occurs in humans, although it occurs in some tissues (especially in the liver).
Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not," "good," and "fold"). Therefore the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome, whereas polyploidy refers to a numerical change in the whole set of chromosomes.
Polyploidy occurs in humans in the form of triploidy (69,XXX) and tetraploidy (92,XXXX), not to be confused with 47,XXX or 48, XXXX aneuploidy. Triploidy, usually due to polyspermy, occurs in about 2-3% of all human pregnancies and ~15% of miscarriages. The vast majority of triploid conceptions end as miscarriage and those that do survive to term typically die shortly after birth. In some cases survival past birth may occur longer if there is mixoploidy with both a diploid and a triploid cell population present. Triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). Diandry is almost always caused by the fertilization of an egg by two sperm (dispermy). Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry appears to predominate among early miscarriages while digyny predominates among triploidy that survives into the fetal period. However, among early miscarriages, digyny is also more common in those cases <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. In digyny there is typically an asymmetric poorly grown fetus, with marked adrenal hypoplasia and a very small placenta. In diandry, the fetus (when present) is typically normally grown or symmetrically growth restricted, with normal adrenal glands and an abnormally large cystic placenta that is called a partial hydatidiform mole. These parent-of-origin effects reflect the effects of genomic imprinting. Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1-2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism. Mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells. Procedure : 1. Keep the onion root tip in 1% colchicine for 10 -24 hrs at 40 C or for 3-6 hrs at room temperature 2. Transfer in Distilled water 3. Then keep the sample in 1N HCl for 10 minutes 4. wash with water 5. excise the root tip portion 6. add few drops of acetocarmine dye and keep for 15 minutes 7. Tease with needle 8. Heat fix by passing throgh the flame 2 – 3 times 9. Cover with cover slip and tap and view under 100x
Observations: 23 chromosomes observed instead of 16, confirming polyploidy.
Precautions: 1. Slide should not be over heated 2. Stain should not be used in excess 3. Tapping should be done gently.
Aim:
To study polytene chromosome using permanent slides.
Theory:
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881. The hereditary nature of these structures was given by Emil Heitz and Hans Bauer in early 1930s by their studies on Drosophila melanogaster.
To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), forming a giant polytene chromosome. Polyteny is achieved by replication of the DNA several times without nuclear division and the resulting daughter chromatids do not separate and remain aligned side by side. A polytene chromosome in the cells of Drosophila salivary glands has about a thousand DNA molecules arranged side by side, all attached at their centromeres, which arise from ten rounds of DNA replication.
Polytene chromosomes occur in salivary gland, trachea, fat cells and malphigian tubules of many insects. They are known to occur in secretary tissues of other dipteran insects such as the Malpighian tubules of Sciara and also in protists, plants, mammals, or in cells from other insects. Some of the largest polytene chromosomes known occur in larval salivary gland cells of the Chironomid genus Axarus.
In polytene cells the chromosomes are visible during interphase. Polytene chromosomes have characteristic light and dark banding patterns. Dark banding frequently corresponds to inactive chromatin, while light banding is usually found at areas with higher transcriptional activity. The chromomeres (regions in which chromatin is more tightly coiled) alternate with regions where the DNA fibers are loosely folded. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin and general chromatin structure. The banding pattern is specific for each pair of homologous chromosomes. They have their own characteristic morphology and position which permits detailed chromosome mapping. The polytene chromosomes consist of coiled or associated homologous pairs of chromosomes. This sort of association is termed as somatic pairing. This permits the identification of abnormalities like deletion, inversions and duplications as regions looped out of the chromosomes.
In addition to increasing the volume of the cell's nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.
The polytene chromosomes develop swellings at particular points. Such chromosomal swellings are called Chromosome Puffs. They are diffuse uncoiled regions of the polytene chromosome that are sites of active RNA transcription and commonly occur in bands. In polytene chromosomes a series of loops may be given out laterally. These loops are called Balbiani Rings, which are large chromosome puffs. They are rich in DNA and mRNA.
Observation:
1. Polytene chromosomes are giant chromosomes with cable like structure. The maternal and paternal homologues remain associated side by side in somatic pairing which have undergone multiple DNA replications.
2. They show characteristic morphology in which dark bands alternate with interbands.
3. The chromosomes also contained swellings or puffs at particular points (mainly in dark band region). These swelling are called Balbiani Rings.
SEX LINKED INHERITANCE
Y-Linked inheritance ( Hollandric Inheritance) :
• Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. • Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. • The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics. • It is passed from father to all sons i.e. 100% male progeny will be affected. • It does not skip generations.
X-Linked Dominant Inheritance :
• Both males and females are affected ; often more females than males are affected • Does not skip generations. Affected sons must have an affected mother: affected daughters must have either an affected mother or an affected father. • Affected fathers will pass the trait on to all their daughters. • Affected mothers (if heterozygous ) will pass the trait on to ½ of their sons and ½ of their daughters. • An example of an X-linked dominant trait in humans is hypophosphatemia or familial vitamin D-resistant rickets. People with this trait have features that superficially resemble those produced by rickets.
X-Linked Recessive Inheritance:
• More males than females are affected. • Affected sons are usually born to unaffected mothers; thus, the trait skips generations. • Approximately ½ of a carrier (heterozygous) mother’s sons are affected. • It is never passed from father to son. • All daughters of affected fathers are carriers. • An example of an X-linked recessive trait in humans is color blindness.
DOWN’S SYNDROME Down’s syndrome is a form of chromosomal variation or aneuploidy characterized by the presence of an extra chromosome 21 in body. Children with Down’s syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of mental retardation. This varies from person to person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with care and support, most children with down syndrome can grow up to have healthy, happy, productive lives. It is named after Langdon Down, who first described its clinical signs in 1866 and formerly known as “Mongolism”. GENOTYPE Male - Full/partial (mosaic) Female -Disorder was identified as chromosome 21 trisomy by Jerome Lejuene in 1959. INCIDENCE RATE The incidence of Down syndrome is estimated at one per 800 to one per 1000 births. The incidence rate increases with increase in maternal age. CAUSES • Maternal age: influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24 , the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. About 75% of these maternal non –disjunctions occur during meiosis I, with the reminder occurring during meiosis II. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of down syndrome manifesting in pregnancies in older mothers. • NON-DISJUNCTION: Non-Disjunction event during gametogenesis or early cell division during embryogenesis may result in full or mosaic Down’s syndrome respectively. Out of 95% of total cases, 88% causes result due to non-disjunction event during female gametogenesis and 8% during gametogenesis in males.
• MOSAICISM: Trisomy 21 is usually caused by non-disjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic. Down syndrome (46, XX/47, XX, +21). This can occur in one of two ways
1. A non-disjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21.
2. Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the causes of 1-2% of the observed Down syndrome
SYMPTOMS:
Most children with Down syndrome have:
• Distinct facial features, such as a flat face, small ears, flat nasal bridge, slanting eyes, oblique parperal fiscers and a small mouth with protruding tongue. • A short neck and short arms and legs. • Excessive space between large toe and second toe. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below average intelligence. • Occipital is flat. • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is highly consistent feature that is helpful in making a diagnosis.
Many children with Down syndrome are also born with heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Luckily, most of these problems can be treated
PATAU’S SYNDROME
Patau’s syndrome, also called trisomy 13, is a congenital disorder associated with the presence of an extra copy of chromosome 13. The extra chromosome 13 causes numerous physical and mental abnormalities, especially heart defects. Patau syndrome is named after Dr. Klaus Patau, who reported the syndrome and its association with trisomy in 1960.
Genetics:
75 to 80% of the cases of Patau syndrome are caused by trisomy of chromosome 13. Some of these cases are the result of a total trisomy, while others are the result of a partial trisomy. Partial trisomy generally causes less severe physical symptoms than full trisomy. 10% of these cases are of the mosaic type, in which only some of the body’s cells have the extra chromosome. The physical symptoms of the mosaic form of Patau syndrome depends on the number and type of cells that carry the trisomy. Most of the cases of trisomy are not passed on from one generation to the next. Usually they result from a malfunction in the cell division (mitosis) that occurs after conception. At least 75% of the cases of Patau syndrome are caused by errors in chromosome replication that occur after conception. The remaining 25% are caused by the inheritance of translocations of chromosome 13 with other chromosomes within the parental chromosomes. In these cases, a portion of another chromosome switches places with a portion of chromosome 13. This leads to errors in the genes on both chromosome 13 and the chromosome from which the translocated portion originated.
Karyotype:
Full trisomy: 47,XX,+13
47,XY,+13
Mosaic trisomy: 47,XX/46,XX,+13
Incidence Rate:
Incidence rate is 1 in 6000 births, births may be falling due to prenatal screening and selective termination of pregnancy. The risk of Patau syndrome seems to increase with the mother’s age, particularly if she is over 30 when pregnant. Male and female children are equally affected, and the syndrome occurs in all races.
Signs and symptoms:
• Mental and motor retardation. • Polidactyly (extra digits) • Microcephaly • Low set ears • Holoprospncephaly (failure of the fore brain two divide properly) • Heart defects (80% of the cases) • Structural eye defects, including Micropthalmia, retinal Dysplasia or retinal detachment, cortical visual loss and optic nerve hypoplasia • Cleft palate or hare lip (not complete fusion of soft and hard palate) • Meningomyelocete (spinal defect or motor abnormality) • Omphalocete (abdominal defect) • Abnormal Genitalia • Abnormal Palm pattern • Overlapping of fingers over thumb • Micrognathia (small jaws) • Hypertonia (low muscle tone) • Cryptorchoidism
The life expectancy is very limited. Most die in the first week and few survive beyond a year. The median age of death is 2.5 days. More than 80% die within a month. Only 5 % last 6 months. The maximum life Span is 21 years. In many cases, spontaneous abortion (miscarriage) occurs and the fetus does not survive to term. In other cases, the affected individual is stillborn.
PHILADELPHIA CHROMOSOME
Philadelphia chromosome is referred to an abnormally short chromosome 22, one of the two chromosomes involved in translocation with chromosome 9 in this condition. This chromosome abnormality causes chronic myeloid leukemia (CML) and is also seen in some cases of acute lymphoblastic leukemia (AML). The Philadelphia chromosome was first discovered and described in 1960 by Peter Nowell from University of Pennsylvania School of medicine and David Hungerford from the Fox Chase Cancer Center’s Institute for cancer research and was therefore named after the city in which both centers were located. The mechanism by which the Philadelphia chromosome arises as a result of translocation was however identified by Janet D. Rowley at the University of Chicago in 1973.
Molecular Genetics:
Due to translocation, a part of bcr (Breakpoint Cluster Region) gene from chromosome 22 gets fused with part of the abl (Abelson) gene on chromosome 9. The abl gene (22, q11) encodes for tyrosine kinase protein in a regulated manner. During reciprocal translocation, parts of these two chromosomes swap pieces, which lead to fusion of abl-bcr on shorter chromosome 22 (Ph chromosome). This fused gene encodes for a 21 kD protein with tyrosine kinase enzyme activity but in uncontrolled amount as regulatory sequences may have been dissociated from gene during translocation. The BCR-ABL protein is expressed at unregulated levels and interacts with interleukin β3 receptor subunit, which in turn activates number of cell cycle controlling proteins thus speeding the cell division. It also results in inhibition of DNA repair, causing genomic instability. The efficacy in CML of a drug that inhibits the BCR-ABL tyrosine kinase has provided the final proof that the BCR-ABL oncoprotein is the unique cause of CML. Nomenclature:
t(9;22)q34;q11) based on International System for Human Cytogenetic Nomenclature.
Incidence Rate:
95% of patients suffering from Chronic Myeloid Leukemia while only 10-12% patients suffering from acute lymphoblast leukemia show this abnormality.
Disease and Symptoms:
Philadelphia chromosome causes Chronic Myeloid Leukemia (CML). This translocation occurs in single bone marrow cell and through process of clonal expansion, gives rise to leukemia. A progressive blood and bone marrow disease usually occurs during or after middle age and rarely in children. Normally the bone marrow makes blood stem cells that develop into mature blood cells after some time. A blood stem cell may become a myeloid stem cell or a lymphoid cell that develops into white blood cell (WBC). In CML, too many blood stem cells develop into WBCs. These WBCs are abnormal and do not develop into healthy WBC’s. These cells build up in bone marrow leaving less room for healthy cells resulting in anemia.
Symptoms:
• Mild to moderate anaemia • Fatigue • Low grade fevers or sweats • Fullness in the abdomen caused by enlarged spleen • left-upper-quadrant pain caused by splenic infarction
Aim: To verify Mendelian law and study deviations from laws.
Observation: The observed ratio of different variety of seeds was 22:7.
Result: Since the observed ratio is close to the phenotypic ratio observed in case of a monohybrid cross, we hypothesize that the above case represents monohybrid cross.
Theory: A Monohybrid cross is a cross between parents who are heterozygous at one locus. It is a breeding experiment dealing with a single character. For example, a monohybrid cross between two pure-breeding plants (homozygous for their respective traits), one with yellow seeds (the dominant trait) and one with green seeds (the recessive trait), would be expected to produce an F1 (first) generation with only yellow seeds because the allele for yellow seeds is dominant to that of green. A monohybrid cross compares only one trait. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%. Maternal B b Paternal B BB Bb b Bb bb It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact. For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross. Discussion: On calculating the chi-square value it was found to be 0.011, when this value was compared with the standard table showing relation between degree of freedom and chi square value for 5% probability it was found to be less. Thus the hypothesis is accepted.
KLINEFELTER’S SYNDROME
Klinefelter’s syndrome is a genetic disorder, which affects males, causing reduced fertility and development of small testicles. Affected individuals have atleast one Y chromosome and atleast two X chromosomes. It is named after Dr. Harry klinefelter , an endocrinologist who first described it in 1942.
Genotype:
It is a condition caused by chromosome nondisjunction in males; affected individuals have a pair of X sex chromosomes instead of just one. About 50-60% of the cases are due to maternal nondisjunction (75% meiosis I errors). Despite the relatively mild phenotypic features of this disorder, it is estimated that at least half of the 47, XXY conceptions are spontaneously aborted. In mammals with more than one X chromosome, the genes on all but one X chromosome are not expressed; this is known as X inactivation. This happens in XXY males as well as normal XX females. A few genes located in the pseudoautosomal regions, however, have corresponding genes on the Y chromosome and are capable of being expressed. These triploid genes in XXY males may be responsible for symptoms associated with Klinefelter's syndrome.
Karyotype: 47,XXY Males
48, XXXY Males
Incidence rate:
The condition is reported in roughly 1 out of every 500 males.
Signs and Symptoms:
• Taller than average individuals with longer arms and legs • Slightly feminized physique with reduced body hair • Small testis (microorchidism). • Serum testosterone levels are lower than normal. • Urinary gonadotropin levels are elevated. • Infertility (azoospermia) may result from atrophy of seminiferous tubules. • Gynecomastia (breast development). • Mild degree of mental retardation. • Language learning impairment but not mentally retarded.
Aim: To study the human karyotype.
Theory:
A karyotype is the characteristic chromosome complement of a eukaryote species.The preparation and study of karyotypes is part of cytogenetics.
In normal diploid organisms, autosomal chromosomes are present in two identical copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is also known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.The chromosomal pairs are arranged in descending order of size with autosomes followed by sex chromosomes.
The study of karyotypes is made possible by staining: usually a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn foetus can be determined by observation of interphase cells .
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for women contain two X chromosomes and are denoted 46,XX; men have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Purpose of Karyotype:
Karyotypes can be used for many purposes:
- to study chromosomal aberrations
- to study cellular function
- to study taxonomic relationships, or
- to gather information about past evolutionary events.
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well suited example is the Philadelphia chromosome, a translocation mutation associated with chronic myelogenous leukemia and less often wuth acute lymphoblastic leukemia.
Observations in a Karyotype:
Six different characteristics of karyotypes are usually observed and compared:
- differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication.
- differences in the position of centromeres. This is brought about by translocations.
- differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
- differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.
- differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
- differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
After sorting by size, chromosomes are further classified as :
Position of the centromere:
Metacentric- having centromere in the centre.
Submetacentric- having centromere near the centre.
Acrocentric- having centromere near the end.
Telocentric- having centromere at the end.
Number of centromeres:
Monocentric- having a single centromere.
Dicentric-having two centromeres.
Polycentric-having many centromeres.
Holocentric-absence of functional centromere.Generally, a dispersed centromere is present over the entire chromosome.
Variation is often found:
- between the two sexes
- between the germ-line and soma (between gametes and the rest of the body)
- between members of a population (chromosome polymorphism)
- geographical variation between races
- mosaics or otherwise abnormal individuals
Human Chromosome Groups:
Group Chromosomes Description A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric B 4,5 Large; submetacentric with two arms very different in size C 6–12,X Medium size; submetacentric D 13–15 Medium size; acrocentric with satellites E 16–18 Small; 16 is metacentric but 17 and 18 are submetacentric F 19,20 Small; metacentric G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the Y Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller than chromosome 22.
Depiction Of Karyotypes: Cytogenetics employs several techniques to visualize different aspects of chromosomes. G-banding :It is obtained with Giemsa stain following digestion of chromosomes with trypsin.It brings out sulfur rich proteins. It yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300-400 bands in a normal, human genome. R-banding :It is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).It requires heat treatment so as to denature the bonds between adenine and thymine (and other regions get stained). C-banding: Giemsa binds to constitutive heterochromatin, so it stains centromeres. Q-banding: It is a fluorescent pattern obtained using quinacrine mustard for staining. The pattern of bands observed using fluorescent microscope is very similar to that seen in G-banding. T-banding: visualize telomeres. In the "classic" karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2) Chromosome abnormalities: Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells. Chromosomal abnormalities that lead to disease in humans include Turner syndrome, Klinefelter syndrome, Edwards syndrome, Down syndrome, Patau syndrome, trisomy 8, trisomy 9 and trisomy 16. Some disorders arise from loss of just a piece of one chromosome, including Cri du chat, 1p36 Deletion syndrome, Angelman syndrome. All such abnormalities can be diagnosed using a karyotype.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: Given ratio of seeds 4 Grams colored seeds 8 Black colored seeds 4 Green colored seeds Result:
Since the observed ratio is close to the phenotypic ratio observed in case of incomplete dominance, we hypothesize that the above case represents incomplete dominance. This case represents a deviation from Mendelian laws.
Theory:
Mendel gave the law of inheritance for Monohybrid as well as Dihybrid cross for F1 & F2 Generation. For a monohybrid cross between RR tall character (dominant) and rr dwarf character (recessive), the mendelian ratio of phenotype for F2 Generation is 3:1 (3 tall, 1 dwarf) and genotypic ratio is 1:2:1.
The incomplete dominance was observed in Antirrhinum majus. The RR-red character, rr-white when crossed the flowers in F1 generation were all pink. When F1 generation flower is self crossed the flowers of F2 generation were 1(red), 2 (pink), 1 (white) which shows that 1:2:1 was followed.
RR rr
F1 generation Rr (all pink)
F2 generation R r
RR Rr
Rr Rr
R
r
RR-red Rr-pink rr-white
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance
Expected ratio: 1:2:1 Observed ratio: 1/4*8 =2 1/2*8=4 1/4*8=2 Degree of freedom for given example (n-1) (3-1)=2 X2 test is applied. X2= (obs-exp)2 /exp + (obs-exp)2 /exp +(obs-exp)2 /exp X2=(4-4)2 /4+ (8-8)2 /8+(4-4)2 /4=0
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis.
EDWARD SYNDROME
The Edward's syndrome, which got its name after the famous doctor, Dr. John Edward (1960), is a genetic chromosomal disorder caused by an error in cell division resulting in an additional third chromosome 18. Edward's syndrome is manifested by a characteristic pattern of anatomical defects in a newborn child and has major implications for its health and survival.
Genetics:
Edward’s syndrome is characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or part (such as due to translocations). In majority of Edward's syndrome cases, all cells of the individual contain additional chromosome 18. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. With a translocation, the person has a partial trisomy for chromosome 18 and the abnormalities are often less than for the typical Edward’s. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edward’s syndrome.
Karyotype: 47,XY,+18
47,XX,+18
46,XX/47,XX,+18 (Mosaic condition)
Incidence Rate: Edward's syndrome is second most common after Down syndrome, occurs in approximately one among 3000 to 6000 births. The incidence rate increases as the mother's age increases. Signs and Symptoms: • Growth Deficiency, • Abnormal skull shape and facial features, • Clenched hands, • Rocker bottom feet, • Cardiac and renal abnormalities, • Horse shoe-shaped kidney, • Low set and deformed ears, • Prominent external genitalia, • Small placenta, • Mental retardation, • Hypotonia, • Microcephaly, • Micronagthia (abnormally small jaw), • Cleft lip, • Respiratory Failure -Apnea The survival rate of Edward’s Syndrome is very low, resulting from heart abnormalities, kidney malformations, and other internal organ disorders. About 95% die in utero. Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life. Major causes of death include apnea and heart abnormalities.
Aim: To verify Mendelian laws and study deviations from laws.
Observation: The observed ratio was 30:2.
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of duplicate genes, we hypothesize that the above case represents duplicate genes.
Theory: Gene duplication (or chromosomal duplication) is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination or duplication of an entire chromosome. The second copy of the gene is often free from mutations . So it will have no deleterious effects to its host organism. The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a different function and/or structure. Gene duplication plays a major role in evolution. Plants are the most prolific genome duplicators. For example, wheat is hexaploid (a kind of polyploid), meaning that it has six copies of its genome. There can be complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene.
e.g. Petal color in snapdragon plant.
P: AABB aabb
(Red colour) (white colour)
F1: AaBb
AaBb AaBb
One allele is sufficient to produce the pigment. Whenever a dominant gene is present, the trait is expressed. The phenotypic ratio observed in F2 generation in this case is 15:1.
Discussion: Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Since, the chi square vale is zero, we accept the abovementioned hypothesis. CRI DU CHAT SYNDROME
Cri du chat syndrome or Lejeune’s syndrome, was first described by Jérôme Lejeune in 1963. It is also called as 5p minus or 5p deletion syndrome. It is a group of symptoms that result from missing a piece of chromosome number 5. The syndrome’s name is based on the infant’s cry, which is high-pitched and sounds like a cat.
Genetics:
Cri du chat syndrome (CdCS) is due to a partial deletion of the short arm of chromosome number 5. Approximately 80% of cases results from a sporadic de novo deletion, while about 10-15% are due to unequal segregation of a parental balanced translocation, where the 5p monosomy is often accompanied by a trisomic portion of the genome. The phenotypes in these individuals may be more severe than in those with isolated monosomy of 5p because of this additional trisomic portion of the genome. Most cases involve terminal deletions with 30-60% loss of 5p material. Fewer than 10% of cases have other rare cytogenetic aberrations (eg, interstitial deletions, mosaicisms, rings and de novo translocations). The deleted chromosome 5 is paternal in origin in about 80% of the cases.
Karyotype: 46, XY, del [5p region]
46, XX, del [5p region]
Incidence Rate: 1 in 20,000 to 50,000 live births
Signs and symptoms:
➢ feeding problems because of difficulty swallowing and sucking ➢ low birth weight and poor growth ➢ severe cognitive, speech, and motor delays ➢ widely set eyes ➢ partial webbing or fusing of fingers or toes ➢ behavioral problems such as hyperactivity, aggression, tantrums ➢ unusual facial features which may change over time ➢ excessive dribbling ➢ constipation ➢ microcephaly(small head) ➢ growth retardation ➢ a round face with full cheeks ➢ low birth weight ➢ hypotonia ➢ epicanthal folds (folds of skin above eyes) ➢ down-slanting palpebral fissures ➢ flat nasal bridge ➢ down-turned mouth and high palate ➢ micrognathia(small jaw) ➢ low-set ears ➢ short fingers ➢ single palmar creases(simian crease) ➢ and cardiac defects(ventricular and atrial septal defects)
The Cri du chat affected people are fertile and can reproduce.
Less frequently encountered findings include:
➢ cleft lip and palate ➢ gut malrotation ➢ inguinal hernia ➢ dislocated hips ➢ cryptorchidism(undescended testis) ➢ rare renal malformations (eg horseshoe kidneys)
Late childhood and adolescence findings include:
➢ severe mental retardation ➢ microcephaly ➢ coarsening of facial features ➢ prominent supraorbital ridges ➢ deep-set eyes ➢ single line on the palm of hand (simian crease) ➢ Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time ➢ In males, testes are often small, but spermatogenesis is thought to be normal.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: 9 red seeds; 6 brown seeds
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of complementary genes, we hypothesize that the above case represents complementary genes. This case represents a deviation from Mendelian laws. Theory: Complementary Genes are those nonallelic genes, which independently show a similar effect but produce a new trait when present together in dominant form. Complementary genes were first studied by Bateson and Punnet (1906) in case of flower color of Sweet Pea (Lathyrus odoratus). Here, the flower color is purple if dominant alleles of two genes are present together (C-P-). The color is white if the double dominant condition is absent (ccP-, c-PP, ccpp ). If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure line, homozygous recessive plant with white flowers, the F1 plant will have colored flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but epistatic interactions of the C and P genes will give a modified 9:7 ratio. The following table describes the interactions for each genotype and how the ratio occurs. Genotype Flower Color Enzyme Activities 9 C_P_ Flowers colored; anthocyanin produced Functional enzymes from both genes 3 C_pp Flowers white; no anthocyanin produced p enzyme non-functional 3 ccP_ Flowers white; no anthocyanin produced c enzyme non-functional 1 ccpp Flowers white; no anthocyanin produced c and p enzymes non-functional
It is believed that the dominant gene C produces an enzyme which converts the raw material into chromatogen. The dominant gene P gives rise to an oxidase enzyme that changes chromatogen into purple anthocyanin pigment. This is confirmed by mixing the extract of the two types of flowerts when purple color is formed. Thus purple color formation is two step reaction and the two genes cooperate to form the ultimate product.
Raw material A Chromagen Anthocyanin
Gene C enz Gene P enz
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Calculated chi square value is 0.07. Degrees of freedom =n-1= 2-1 =1 Chi square value from the table is 3.84
0.07<3.84
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis. DOWN’S SYNDROME
Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is the best known and most common chromosome-related disease syndrome formerly known as “mongolism”. It is named after Langdon Down, who first described its clinical signs in 1866.
Genetics:
About 95% of the cases are caused by non-disjunction, with most of the remainder being caused by chromosome translocation. The extra chromosome is contributed by the mother in 90-95% cases. About 75% of these maternal non-disjunctions occur during meiosis I, with the remainder occurring during meiosis II. There is a strong correlation between maternal age and the risk of producing a child with Down’s syndrome. Some patients with Down’s syndrome have a total of 46 chromosomes instead of 47, but in such cases a translocation has joined a part of long arm of chromosome 21 with the long arm of chromosome 14 (14q21q). Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic Down syndrome. This can occur in one of two ways: • a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21. • a Down’s syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
Karyotype: 47,XY,+21
47,XX,+21
46,XX/47,XX,+21 (Mosaic condition)
Incidence Rate: Down’s syndrome is the most common autosomal aneuploidy seen among live births. It is seen in approximately 1/700 live birth, making it the most common aneuploidy condition compatible with survival to term. The risk for mothers less than 25 years of age to have the trisomy is about 1/1500; at 40 years of age, 1/1000; at 45, 1/400. Pregnant women over 45 are a special high-risk group.
Signs and Symptoms:
• Distinct facial features, such as a flat face, small ears, slanting eyes, and a small mouth. • A short neck and short arms and legs. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below-average intelligence. • Occipital is flat • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is a highly consistent feature that is helpful in making a diagnosis. • Fertility amongst both males and females is reduced. Approx 75% of trisomy 21 conceptions are spontaneously aborted, the Down’s syndrome female’s risk of producing affected live-born offspring is considerably lower than 50%. • Heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Most of these problems can be treated.
XYY SYNDROME
XYY syndrome or Jacob's syndrome is a rare chromosomal disorder that affects males. It is caused by an aneuploidy (trisomy) of the sex chromosomes, Y chromosome, thus a human male receives an extra Y chromosome in each cell. The first published report of a man with a 47,XYY Karyotype was by Avery A. Sandberg and colleagues at Roswell Park Memorial Institute in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence that was karyotyped because he had a daughter with Down syndrome.
Genetics:
Males normally have one X and one Y chromosome. However, individuals with Jacob's syndrome have one X and two Y chromosome. Males with Jacob's syndrome, also called XYY males. 47, XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in chromosome separation during metaphase I or metaphase II called no disjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells.
In some cases, the addition of an extra Y chromosome results from non disjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics
Karyotype: 47, XYY.
46, XY/47, XYY mosaics
Incidence Rate: About 1 in 1,000 boys are born with a 47,XYY Karyotype. The incidence of 47,XYY is not affected by advanced paternal or maternal age. Signs and Symptoms: Physical traits: Most often, the extra Y chromosome causes no unusual physical features or medical problems. • 47, XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height. • Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47, XYY. Testosterone levels (prenatally and postnatally) are normal in 47, XYY males. Most 47, XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason. Behavioral traits: • 47, XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. • As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings • Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47, XYY and are managed no differently than in 46,XY males. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
TRIPLE X SYNDROME
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The first published report of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland, in 1959.
Genetics: The condition is also known as triplo-X, trisomy X, XXX syndrome, and 47,XXX aneuploidy. Unlike other chromosonal conditions (such as fragile X), there is usually no distinguishable difference between women with triple X and the rest of the female population. Triple X syndrome is usually not inherited, but occurs as a random event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the nondisjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of her cells. In some cases, trisomy X occurs during cell division in early embryonic development. The additional X chromosome can come from either the maternal or paternal side. The condition is verified only by karyotype testing as it may not be distinguishable phenotypically. • Full trisomy: In this case, trisomy X occurs during gamete formation. • Mosaic trisomy: Some females with triple X syndrome have an extra X chromosome in only some of their cells. In this case, trisomy X occurs during embryogenesis.
Karyotype:
• Full trisomy: 47,XXX • Mosaic: 46,XX/47,XXX Incidence Rate: Triple X syndrome occurs in around 1 in 1,000 live female births. Signs and symptoms: • Due to inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell and two Barr bodies are visible in somatic cell . Thus, triple X syndrome most often causes no unusual physical features or medical problems. • Females with the condition may have menstrual irregularities. • Although they rarely exhibit severe mental impairments, they have an increased risk of learning disabilities, delayed speech, and language skills. • a lanky/youthful appearance with increased facial beauty has been described, or in some instances varying degrees of androgeny, but these cases usually reflect traits present in near relatives. • Most women with triple X have normal sexual development and are able to conceive children. • A few may experience an early onset of menstruation. • Early menopause. • Triple X women are rarely diagnosed, apart from pre-natal testing methods, such as amniocentesis. Most medical professionals do not regard the condition a disability. However, such status can be sought by parents for early intervention treatment if mild delays are present.
TURNER’S SYNDROME
The syndrome is named after Henry Turner, an Oklahoma endocrinologist, who described it in 1938. The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London. It was found in a 14-year-old girl with signs of Turner syndrome.
Genetics: Turner’s syndrome encompasses several conditions, of which monosomy XO is the most common. Instead of the normal XX sex chromosomes for a female, (or XY for a normal male) only one X chromosome is present and fully functional; in rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). In Turner’ssyndrome, female sexual characteristics are present but generally underdeveloped. The risk factors for Turner’s syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome. There is currently no known cause for Turner syndrome, though there are several theories surrounding the subject. Karyotype: • Full Monosomy: 45,XO • Mosaic Monosomy: 46,XX/45,XO Incidence Rate: Approximately 98% of all fetuses with Turner syndrome result in miscarriage. Turner syndrome accounts for about 10% of the total number of spontaneous abortions in the United States. The incidence of Turner syndrome in live female births is believed to be 1 in 2500. Signs and Symptoms: -Short stature -Lymphedema (swelling) of the hands and feet -Broad chest , poor breast development and widely-spaced nipples -Low hairline -Low-set ears, hearing loss -Reproductive sterility, rudimentary ovaries, gonadal streak (underdeveloped gonadal structures) -Amenorrhea or the absence of a menstrual period -Increased weight, obesity -Shortened metacarpal IV (of hand) -Characteristic facial features,visual impairments -Webbing of the neck (webbed neck) -Congenital heart disease-Coarctation of the aorta -Horseshoe kidney -Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. -Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. -Approximately one-third of all women with Turner syndrome have a thyroid disorder. -Moderately increased risk of developing diabetes. -Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. -Women with Turner syndrome are almost universally infertile. Even when pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner’s Syndrome or Down’s Syndrome. -Other symptoms may include a small lower jaw (micrognathia), cubitus valgus (turned-out elbows), soft upturned nails, palmar crease and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition, and no two individuals will share the same symptoms.
AIM: To prove monogenic inheritance using the result of PTC (phenyl thiocarbamide) test, thus confirming the Hardy Weinberg’s Law.
REQUIREMENTS: Beaker, PTC, alcohol, strips of Whatmann’s paper, oven.
THEORY:
This law was proposed by G.H. Hardy and W.Weinberg independently in 1908. They reported that under certain conditions hereditary conservation of genes is the characteristic of population. This is a static condition and the population is non evolving. Such a kind of stability at the genetic level is called genetic equilibrium.
Hardy Weinberg Law: the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when-
• The population is large enough so that random sampling errors do not affect the allele frequency.
• Mating takes place at random.
• Mutation does not take place or if it does the rate is same in both directions
• All members of population survive and have equal reproductive rates.
• There should be no migration among different populations.
In reality no population satisfies Hardy Weinberg’s Law. But in large natural populations with little migration and negligible natural selection, the Hardy Weinberg Law may be nearly approximated.
Deviations from Hardy Weinberg Law:
Mutations: they produce alternate alleles at a given locus and alter the phenotype. In a population, mutation disturbs the genetic equilibrium and gene frequency. Thus gene pool gradually changes with mutant gene appearing with greater frequency.
Selection: if a mutation is advantageous then natural selection may preserve it and such characters get inherited and contribute more to the gene pool of next generation. Thus genetic equilibrium is changed. e.g. heterozygous advantage in case of sickle cell anemia.
Non random mating: Differential mating is one of the evolutionary forces resulting in abundance of certain genotype at the cost of others.
Small population size and genetic drift: the random change in gene frequency occurring by chance and not under the control of natural selection is a genetic drift. In populations genetic drift favors either a loss or fixation of an allele. The rate at which an allele is lost or becomes fixed, depends on the population size.
Significance of the Hardy Weinberg Law: It provides a tool which helps the population geneticists to determine the degree of evolutionary change by comparing allele frequencies at starting point and at some future point. By this law we can calculate the frequencies of homozygous dominant, homozygous recessive and heterozygous carriers in a population.
PROCEDURE: 1. In a beaker, 1% solution of PTC was prepared in 100% alcohol. 2. Several small strips of Whatman’s paper were dipped in at least for 2-3 hours. 3. The strips were allowed to dry in an oven for about half an hour. 4. After drying, the strips were tasted. OBSERVATION:
No. of positive tasters No. of non tasters population size 17 2 19
CALCULATION: According to Hardy Weinberg Law :
p2 + q2 + 2pq = 1
Where, p = allelic frequency of dominant
q = allelic frequency of recessive
2pq = heterozygous dominant frequency
p2 = homozygous dominant frequency
q2 = homozygous recessive frequency
Phenotypic frequency for the population-
Dominant = P2 + 2pq = 17/19 = 0.894
Recessive = q2 = 2/19 = 0.106
Therefore, q = 0.325
And as we know that p+q = 1 ;
P = 1- q = 0.675
Allelic frequency-
Dominant: p = 0.675
Recessive: q = 0.325
Genotypic frequency for population:
Homozygous dominant: p2 = (0.675)2 = 0.45
Homozygous recessive: q2 = (0.325)2 = 0.10
Heterozygous dominant: 2pq = 0.44
RESULT: Monogenetic inheritance in man was proved using result of PTC, thus confirming HARDY-WEINBERG’S LAW
PRECAUTIONS: 1. Whatman’s strips should be dipped in PTC solution for at least 2-3 hours. 2. The strips should always be properly dried before use. 3. Results should be accurately observed. 4. PTC solution was finely prepared
Aim : To calculate linkage number .
Problem 1:
A cross is made between homozygous wild-type female Drosophila (a+a+b+b+ c+c+) and triple-mutant males (aa bb cc) (the order here is arbitrary). Give the constitution of gametes produced by the parents and also genotype of F1 . A test cross of the F1 gave the following phenotypes:
“a+ b c” 18 “a b+ c” 112 “a b c” 308 “a+ b+ c” 66 “a b c+” 59 “a+ b+ c+” 321 “a+ b c+” 102 “a b+ c+” 15
1000
1. Calculate the percentage of crossover between 3 gene pairs 2. Calculate the percentage of double crossover and coefficient of coincidence 3. Make a linkage map of these genes
Problem 2:
A cross was made between C/C , sh/sh , Wx/Wx stocks of maize and one with the genotype c/c , Sh/Sh , wx/wx. Give the constitution of the gametes produced by theparents and the genotype of F1 A test cross of F1 gave the following phenotype :
C sh Wx 2777 c Sh wx 2708 C Sh wx 116 c sh Wx 122 C sh wx 643 c Sh Wx 626 C Sh Wx 4 c sh wx 3 TOTAL 7000
1) Calculate the percentage of crossover between 3 gene pairs 2) Calculate the percentage of double crossover and coefficient of coincidence 3) Make a linkage map of these genes
PENETRANCE
• It is the frequency with which a heritable trait is manifested by individuals carrying the principal gene or genes conditioning it.
• In other words, the percentage of individuals that show at leastr some degree of expression of a mutant genotype defines the penetrance.
• The presence of a gene may not result in a detectable phenotype.
Example:-
The phenotypic expression of many mutant alleles in drosphila is indistinguishable from wild type.If 15 percent of mutant flies show the wild type appearance ,the mutant gene is said to have a penetrance of 85 percent.
VARIABLE EXPRESSIVITY
• Expressivity reflects the range of expression of the mutant genotype. • In other words it is the variation in the degree to which gene is expressed. • the variability with which basic patterns of inheritance are modified, both in degree and in variety, by the effect of a given gene in people of the same genotype.
Example:-
polydactyly may be expressed as extra toes in one generation and extra fingers in another.
AIM : To Study Polyploidy In Allium cepa Root Tip
Things Required : Slide, Colchicine, 1N HCl, Acetocarmine Dye, Onion Root Tip, needle, forceps, Theory : Polyploidy occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploidy is a state different from most organisms which are normally diploid meaning they have only two sets of chromosomes - one set inherited from each parent; polyploidy may occur due to abnormal cell division. It is most commonly found in plants.
Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see Hibiscus Rosa-Sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U. The occurrence of polyploidy is a mechanism of speciation and is known to have resulted in new species of the plant Salsify (also known as "goatsbeard").
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. Polyploidy can be induced in cell culture by some chemicals: the best known is colchicines, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
Polyploid types are labelled according to the number of chromosome sets in the nucleus: • triploid (three sets; 3x), for example the phylum Tardigrada • tetraploid (four sets; 4x), for example Salmonidae fish • pentaploid (five sets; 5x) • hexaploid (six sets; 6x), for example wheat, kiwifruit • oktoploid (eight sets; 8x), for example Acipenser (genus of sturgeon fish) • dekaploid (ten sets; 10x), for example certain strawberries • dodecaploid (twelve sets; 12x), for example the plant Celosia argentea
Autopolyploidy Autopolyploids are polyploids with multiple chromosome sets derived from a single species. Autopolyploids can arise from a spontaneous, naturally occurring genome doubling (for example, the potato). Others might form following fusion of 2n gametes (unreduced gametes). Bananas and apples can be found as triploid autopolyploids. Autopolyploid plants typically display polysomic inheritance, and are therefore often infertile and propagated clonally Allopolyploidy Allopolyploids are polyploids with chromosomes derived from different species. Triticale is an example of an allopolyploid, having six chromosome sets, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid. Some of the best examples of allopolyploids come from the Brassicas, and the Triangle of U describes the relationships among the three common diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploids. Polyploidy in plants Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (eg. potato) and allopolyploids (eg. canola, wheat, cotton) can be found among both wild and domesticated plant species. Most polyploids display heterosis relative to their parental species, and may display novel variation or morphologies that may contribute to the processes of speciation and eco-niche exploitation The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels. Many of these rapid changes may contribute to reproductive isolation and speciation. There are few naturally occurring polyploid conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n=6x=66), although the origin is unclear. Polyploid crops Polyploid plants tend to be larger and better at flourishing in early succession habitats such as farm fields in the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops (and agricultural weeds) may have unintentionally been bred to a higher level of ploidy. The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. In some situations polyploid crops are preferred because they are sterile. For example many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine. Examples of Polyploid Crops • Triploid crops: banana, apple, ginger, watermelon, citrus • Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek • Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit • Octaploid crops: strawberry, dahlia, pansies, sugar cane Polyploidy in animals Examples in animals are more common in the 'lower' forms such as flatworms, leeches, and brine shrimp. Polyploid animals are often sterile, so they often reproduce by parthenogenesis. Polyploid salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death. One of the only known exceptions to this 'rule' is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae). This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of polyploidy in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.. Polyploidy in humans (Aneuploidy) True polyploidy rarely occurs in humans, although it occurs in some tissues (especially in the liver).
Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not," "good," and "fold"). Therefore the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome, whereas polyploidy refers to a numerical change in the whole set of chromosomes.
Polyploidy occurs in humans in the form of triploidy (69,XXX) and tetraploidy (92,XXXX), not to be confused with 47,XXX or 48, XXXX aneuploidy. Triploidy, usually due to polyspermy, occurs in about 2-3% of all human pregnancies and ~15% of miscarriages. The vast majority of triploid conceptions end as miscarriage and those that do survive to term typically die shortly after birth. In some cases survival past birth may occur longer if there is mixoploidy with both a diploid and a triploid cell population present. Triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). Diandry is almost always caused by the fertilization of an egg by two sperm (dispermy). Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry appears to predominate among early miscarriages while digyny predominates among triploidy that survives into the fetal period. However, among early miscarriages, digyny is also more common in those cases <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. In digyny there is typically an asymmetric poorly grown fetus, with marked adrenal hypoplasia and a very small placenta. In diandry, the fetus (when present) is typically normally grown or symmetrically growth restricted, with normal adrenal glands and an abnormally large cystic placenta that is called a partial hydatidiform mole. These parent-of-origin effects reflect the effects of genomic imprinting. Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1-2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism. Mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells. Procedure : 1. Keep the onion root tip in 1% colchicine for 10 -24 hrs at 40 C or for 3-6 hrs at room temperature 2. Transfer in Distilled water 3. Then keep the sample in 1N HCl for 10 minutes 4. wash with water 5. excise the root tip portion 6. add few drops of acetocarmine dye and keep for 15 minutes 7. Tease with needle 8. Heat fix by passing throgh the flame 2 – 3 times 9. Cover with cover slip and tap and view under 100x
Observations: 23 chromosomes observed instead of 16, confirming polyploidy.
Precautions: 1. Slide should not be over heated 2. Stain should not be used in excess 3. Tapping should be done gently.
Aim:
To study polytene chromosome using permanent slides.
Theory:
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881. The hereditary nature of these structures was given by Emil Heitz and Hans Bauer in early 1930s by their studies on Drosophila melanogaster.
To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), forming a giant polytene chromosome. Polyteny is achieved by replication of the DNA several times without nuclear division and the resulting daughter chromatids do not separate and remain aligned side by side. A polytene chromosome in the cells of Drosophila salivary glands has about a thousand DNA molecules arranged side by side, all attached at their centromeres, which arise from ten rounds of DNA replication.
Polytene chromosomes occur in salivary gland, trachea, fat cells and malphigian tubules of many insects. They are known to occur in secretary tissues of other dipteran insects such as the Malpighian tubules of Sciara and also in protists, plants, mammals, or in cells from other insects. Some of the largest polytene chromosomes known occur in larval salivary gland cells of the Chironomid genus Axarus.
In polytene cells the chromosomes are visible during interphase. Polytene chromosomes have characteristic light and dark banding patterns. Dark banding frequently corresponds to inactive chromatin, while light banding is usually found at areas with higher transcriptional activity. The chromomeres (regions in which chromatin is more tightly coiled) alternate with regions where the DNA fibers are loosely folded. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin and general chromatin structure. The banding pattern is specific for each pair of homologous chromosomes. They have their own characteristic morphology and position which permits detailed chromosome mapping. The polytene chromosomes consist of coiled or associated homologous pairs of chromosomes. This sort of association is termed as somatic pairing. This permits the identification of abnormalities like deletion, inversions and duplications as regions looped out of the chromosomes.
In addition to increasing the volume of the cell's nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.
The polytene chromosomes develop swellings at particular points. Such chromosomal swellings are called Chromosome Puffs. They are diffuse uncoiled regions of the polytene chromosome that are sites of active RNA transcription and commonly occur in bands. In polytene chromosomes a series of loops may be given out laterally. These loops are called Balbiani Rings, which are large chromosome puffs. They are rich in DNA and mRNA.
Observation:
1. Polytene chromosomes are giant chromosomes with cable like structure. The maternal and paternal homologues remain associated side by side in somatic pairing which have undergone multiple DNA replications.
2. They show characteristic morphology in which dark bands alternate with interbands.
3. The chromosomes also contained swellings or puffs at particular points (mainly in dark band region). These swelling are called Balbiani Rings.
SEX LINKED INHERITANCE
Y-Linked inheritance ( Hollandric Inheritance) :
• Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. • Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. • The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics. • It is passed from father to all sons i.e. 100% male progeny will be affected. • It does not skip generations.
X-Linked Dominant Inheritance :
• Both males and females are affected ; often more females than males are affected • Does not skip generations. Affected sons must have an affected mother: affected daughters must have either an affected mother or an affected father. • Affected fathers will pass the trait on to all their daughters. • Affected mothers (if heterozygous ) will pass the trait on to ½ of their sons and ½ of their daughters. • An example of an X-linked dominant trait in humans is hypophosphatemia or familial vitamin D-resistant rickets. People with this trait have features that superficially resemble those produced by rickets.
X-Linked Recessive Inheritance:
• More males than females are affected. • Affected sons are usually born to unaffected mothers; thus, the trait skips generations. • Approximately ½ of a carrier (heterozygous) mother’s sons are affected. • It is never passed from father to son. • All daughters of affected fathers are carriers. • An example of an X-linked recessive trait in humans is color blindness.
DOWN’S SYNDROME Down’s syndrome is a form of chromosomal variation or aneuploidy characterized by the presence of an extra chromosome 21 in body. Children with Down’s syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of mental retardation. This varies from person to person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with care and support, most children with down syndrome can grow up to have healthy, happy, productive lives. It is named after Langdon Down, who first described its clinical signs in 1866 and formerly known as “Mongolism”. GENOTYPE Male - Full/partial (mosaic) Female -Disorder was identified as chromosome 21 trisomy by Jerome Lejuene in 1959. INCIDENCE RATE The incidence of Down syndrome is estimated at one per 800 to one per 1000 births. The incidence rate increases with increase in maternal age. CAUSES • Maternal age: influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24 , the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. About 75% of these maternal non –disjunctions occur during meiosis I, with the reminder occurring during meiosis II. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of down syndrome manifesting in pregnancies in older mothers. • NON-DISJUNCTION: Non-Disjunction event during gametogenesis or early cell division during embryogenesis may result in full or mosaic Down’s syndrome respectively. Out of 95% of total cases, 88% causes result due to non-disjunction event during female gametogenesis and 8% during gametogenesis in males.
• MOSAICISM: Trisomy 21 is usually caused by non-disjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic. Down syndrome (46, XX/47, XX, +21). This can occur in one of two ways
1. A non-disjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21.
2. Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the causes of 1-2% of the observed Down syndrome
SYMPTOMS:
Most children with Down syndrome have:
• Distinct facial features, such as a flat face, small ears, flat nasal bridge, slanting eyes, oblique parperal fiscers and a small mouth with protruding tongue. • A short neck and short arms and legs. • Excessive space between large toe and second toe. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below average intelligence. • Occipital is flat. • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is highly consistent feature that is helpful in making a diagnosis.
Many children with Down syndrome are also born with heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Luckily, most of these problems can be treated
PATAU’S SYNDROME
Patau’s syndrome, also called trisomy 13, is a congenital disorder associated with the presence of an extra copy of chromosome 13. The extra chromosome 13 causes numerous physical and mental abnormalities, especially heart defects. Patau syndrome is named after Dr. Klaus Patau, who reported the syndrome and its association with trisomy in 1960.
Genetics:
75 to 80% of the cases of Patau syndrome are caused by trisomy of chromosome 13. Some of these cases are the result of a total trisomy, while others are the result of a partial trisomy. Partial trisomy generally causes less severe physical symptoms than full trisomy. 10% of these cases are of the mosaic type, in which only some of the body’s cells have the extra chromosome. The physical symptoms of the mosaic form of Patau syndrome depends on the number and type of cells that carry the trisomy. Most of the cases of trisomy are not passed on from one generation to the next. Usually they result from a malfunction in the cell division (mitosis) that occurs after conception. At least 75% of the cases of Patau syndrome are caused by errors in chromosome replication that occur after conception. The remaining 25% are caused by the inheritance of translocations of chromosome 13 with other chromosomes within the parental chromosomes. In these cases, a portion of another chromosome switches places with a portion of chromosome 13. This leads to errors in the genes on both chromosome 13 and the chromosome from which the translocated portion originated.
Karyotype:
Full trisomy: 47,XX,+13
47,XY,+13
Mosaic trisomy: 47,XX/46,XX,+13
Incidence Rate:
Incidence rate is 1 in 6000 births, births may be falling due to prenatal screening and selective termination of pregnancy. The risk of Patau syndrome seems to increase with the mother’s age, particularly if she is over 30 when pregnant. Male and female children are equally affected, and the syndrome occurs in all races.
Signs and symptoms:
• Mental and motor retardation. • Polidactyly (extra digits) • Microcephaly • Low set ears • Holoprospncephaly (failure of the fore brain two divide properly) • Heart defects (80% of the cases) • Structural eye defects, including Micropthalmia, retinal Dysplasia or retinal detachment, cortical visual loss and optic nerve hypoplasia • Cleft palate or hare lip (not complete fusion of soft and hard palate) • Meningomyelocete (spinal defect or motor abnormality) • Omphalocete (abdominal defect) • Abnormal Genitalia • Abnormal Palm pattern • Overlapping of fingers over thumb • Micrognathia (small jaws) • Hypertonia (low muscle tone) • Cryptorchoidism
The life expectancy is very limited. Most die in the first week and few survive beyond a year. The median age of death is 2.5 days. More than 80% die within a month. Only 5 % last 6 months. The maximum life Span is 21 years. In many cases, spontaneous abortion (miscarriage) occurs and the fetus does not survive to term. In other cases, the affected individual is stillborn.
PHILADELPHIA CHROMOSOME
Philadelphia chromosome is referred to an abnormally short chromosome 22, one of the two chromosomes involved in translocation with chromosome 9 in this condition. This chromosome abnormality causes chronic myeloid leukemia (CML) and is also seen in some cases of acute lymphoblastic leukemia (AML). The Philadelphia chromosome was first discovered and described in 1960 by Peter Nowell from University of Pennsylvania School of medicine and David Hungerford from the Fox Chase Cancer Center’s Institute for cancer research and was therefore named after the city in which both centers were located. The mechanism by which the Philadelphia chromosome arises as a result of translocation was however identified by Janet D. Rowley at the University of Chicago in 1973.
Molecular Genetics:
Due to translocation, a part of bcr (Breakpoint Cluster Region) gene from chromosome 22 gets fused with part of the abl (Abelson) gene on chromosome 9. The abl gene (22, q11) encodes for tyrosine kinase protein in a regulated manner. During reciprocal translocation, parts of these two chromosomes swap pieces, which lead to fusion of abl-bcr on shorter chromosome 22 (Ph chromosome). This fused gene encodes for a 21 kD protein with tyrosine kinase enzyme activity but in uncontrolled amount as regulatory sequences may have been dissociated from gene during translocation. The BCR-ABL protein is expressed at unregulated levels and interacts with interleukin β3 receptor subunit, which in turn activates number of cell cycle controlling proteins thus speeding the cell division. It also results in inhibition of DNA repair, causing genomic instability. The efficacy in CML of a drug that inhibits the BCR-ABL tyrosine kinase has provided the final proof that the BCR-ABL oncoprotein is the unique cause of CML. Nomenclature:
t(9;22)q34;q11) based on International System for Human Cytogenetic Nomenclature.
Incidence Rate:
95% of patients suffering from Chronic Myeloid Leukemia while only 10-12% patients suffering from acute lymphoblast leukemia show this abnormality.
Disease and Symptoms:
Philadelphia chromosome causes Chronic Myeloid Leukemia (CML). This translocation occurs in single bone marrow cell and through process of clonal expansion, gives rise to leukemia. A progressive blood and bone marrow disease usually occurs during or after middle age and rarely in children. Normally the bone marrow makes blood stem cells that develop into mature blood cells after some time. A blood stem cell may become a myeloid stem cell or a lymphoid cell that develops into white blood cell (WBC). In CML, too many blood stem cells develop into WBCs. These WBCs are abnormal and do not develop into healthy WBC’s. These cells build up in bone marrow leaving less room for healthy cells resulting in anemia.
Symptoms:
• Mild to moderate anaemia • Fatigue • Low grade fevers or sweats • Fullness in the abdomen caused by enlarged spleen • left-upper-quadrant pain caused by splenic infarction
Aim: To verify Mendelian law and study deviations from laws.
Observation: The observed ratio of different variety of seeds was 22:7.
Result: Since the observed ratio is close to the phenotypic ratio observed in case of a monohybrid cross, we hypothesize that the above case represents monohybrid cross.
Theory: A Monohybrid cross is a cross between parents who are heterozygous at one locus. It is a breeding experiment dealing with a single character. For example, a monohybrid cross between two pure-breeding plants (homozygous for their respective traits), one with yellow seeds (the dominant trait) and one with green seeds (the recessive trait), would be expected to produce an F1 (first) generation with only yellow seeds because the allele for yellow seeds is dominant to that of green. A monohybrid cross compares only one trait. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%. Maternal B b Paternal B BB Bb b Bb bb It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact. For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross. Discussion: On calculating the chi-square value it was found to be 0.011, when this value was compared with the standard table showing relation between degree of freedom and chi square value for 5% probability it was found to be less. Thus the hypothesis is accepted.
KLINEFELTER’S SYNDROME
Klinefelter’s syndrome is a genetic disorder, which affects males, causing reduced fertility and development of small testicles. Affected individuals have atleast one Y chromosome and atleast two X chromosomes. It is named after Dr. Harry klinefelter , an endocrinologist who first described it in 1942.
Genotype:
It is a condition caused by chromosome nondisjunction in males; affected individuals have a pair of X sex chromosomes instead of just one. About 50-60% of the cases are due to maternal nondisjunction (75% meiosis I errors). Despite the relatively mild phenotypic features of this disorder, it is estimated that at least half of the 47, XXY conceptions are spontaneously aborted. In mammals with more than one X chromosome, the genes on all but one X chromosome are not expressed; this is known as X inactivation. This happens in XXY males as well as normal XX females. A few genes located in the pseudoautosomal regions, however, have corresponding genes on the Y chromosome and are capable of being expressed. These triploid genes in XXY males may be responsible for symptoms associated with Klinefelter's syndrome.
Karyotype: 47,XXY Males
48, XXXY Males
Incidence rate:
The condition is reported in roughly 1 out of every 500 males.
Signs and Symptoms:
• Taller than average individuals with longer arms and legs • Slightly feminized physique with reduced body hair • Small testis (microorchidism). • Serum testosterone levels are lower than normal. • Urinary gonadotropin levels are elevated. • Infertility (azoospermia) may result from atrophy of seminiferous tubules. • Gynecomastia (breast development). • Mild degree of mental retardation. • Language learning impairment but not mentally retarded.
Aim: To study the human karyotype.
Theory:
A karyotype is the characteristic chromosome complement of a eukaryote species.The preparation and study of karyotypes is part of cytogenetics.
In normal diploid organisms, autosomal chromosomes are present in two identical copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is also known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.The chromosomal pairs are arranged in descending order of size with autosomes followed by sex chromosomes.
The study of karyotypes is made possible by staining: usually a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn foetus can be determined by observation of interphase cells .
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for women contain two X chromosomes and are denoted 46,XX; men have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Purpose of Karyotype:
Karyotypes can be used for many purposes:
- to study chromosomal aberrations
- to study cellular function
- to study taxonomic relationships, or
- to gather information about past evolutionary events.
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well suited example is the Philadelphia chromosome, a translocation mutation associated with chronic myelogenous leukemia and less often wuth acute lymphoblastic leukemia.
Observations in a Karyotype:
Six different characteristics of karyotypes are usually observed and compared:
- differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication.
- differences in the position of centromeres. This is brought about by translocations.
- differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
- differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.
- differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
- differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
After sorting by size, chromosomes are further classified as :
Position of the centromere:
Metacentric- having centromere in the centre.
Submetacentric- having centromere near the centre.
Acrocentric- having centromere near the end.
Telocentric- having centromere at the end.
Number of centromeres:
Monocentric- having a single centromere.
Dicentric-having two centromeres.
Polycentric-having many centromeres.
Holocentric-absence of functional centromere.Generally, a dispersed centromere is present over the entire chromosome.
Variation is often found:
- between the two sexes
- between the germ-line and soma (between gametes and the rest of the body)
- between members of a population (chromosome polymorphism)
- geographical variation between races
- mosaics or otherwise abnormal individuals
Human Chromosome Groups:
Group Chromosomes Description A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric B 4,5 Large; submetacentric with two arms very different in size C 6–12,X Medium size; submetacentric D 13–15 Medium size; acrocentric with satellites E 16–18 Small; 16 is metacentric but 17 and 18 are submetacentric F 19,20 Small; metacentric G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the Y Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller than chromosome 22.
Depiction Of Karyotypes: Cytogenetics employs several techniques to visualize different aspects of chromosomes. G-banding :It is obtained with Giemsa stain following digestion of chromosomes with trypsin.It brings out sulfur rich proteins. It yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300-400 bands in a normal, human genome. R-banding :It is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).It requires heat treatment so as to denature the bonds between adenine and thymine (and other regions get stained). C-banding: Giemsa binds to constitutive heterochromatin, so it stains centromeres. Q-banding: It is a fluorescent pattern obtained using quinacrine mustard for staining. The pattern of bands observed using fluorescent microscope is very similar to that seen in G-banding. T-banding: visualize telomeres. In the "classic" karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2) Chromosome abnormalities: Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells. Chromosomal abnormalities that lead to disease in humans include Turner syndrome, Klinefelter syndrome, Edwards syndrome, Down syndrome, Patau syndrome, trisomy 8, trisomy 9 and trisomy 16. Some disorders arise from loss of just a piece of one chromosome, including Cri du chat, 1p36 Deletion syndrome, Angelman syndrome. All such abnormalities can be diagnosed using a karyotype.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: Given ratio of seeds 4 Grams colored seeds 8 Black colored seeds 4 Green colored seeds Result:
Since the observed ratio is close to the phenotypic ratio observed in case of incomplete dominance, we hypothesize that the above case represents incomplete dominance. This case represents a deviation from Mendelian laws.
Theory:
Mendel gave the law of inheritance for Monohybrid as well as Dihybrid cross for F1 & F2 Generation. For a monohybrid cross between RR tall character (dominant) and rr dwarf character (recessive), the mendelian ratio of phenotype for F2 Generation is 3:1 (3 tall, 1 dwarf) and genotypic ratio is 1:2:1.
The incomplete dominance was observed in Antirrhinum majus. The RR-red character, rr-white when crossed the flowers in F1 generation were all pink. When F1 generation flower is self crossed the flowers of F2 generation were 1(red), 2 (pink), 1 (white) which shows that 1:2:1 was followed.
RR rr
F1 generation Rr (all pink)
F2 generation R r
RR Rr
Rr Rr
R
r
RR-red Rr-pink rr-white
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance
Expected ratio: 1:2:1 Observed ratio: 1/4*8 =2 1/2*8=4 1/4*8=2 Degree of freedom for given example (n-1) (3-1)=2 X2 test is applied. X2= (obs-exp)2 /exp + (obs-exp)2 /exp +(obs-exp)2 /exp X2=(4-4)2 /4+ (8-8)2 /8+(4-4)2 /4=0
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis.
EDWARD SYNDROME
The Edward's syndrome, which got its name after the famous doctor, Dr. John Edward (1960), is a genetic chromosomal disorder caused by an error in cell division resulting in an additional third chromosome 18. Edward's syndrome is manifested by a characteristic pattern of anatomical defects in a newborn child and has major implications for its health and survival.
Genetics:
Edward’s syndrome is characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or part (such as due to translocations). In majority of Edward's syndrome cases, all cells of the individual contain additional chromosome 18. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. With a translocation, the person has a partial trisomy for chromosome 18 and the abnormalities are often less than for the typical Edward’s. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edward’s syndrome.
Karyotype: 47,XY,+18
47,XX,+18
46,XX/47,XX,+18 (Mosaic condition)
Incidence Rate: Edward's syndrome is second most common after Down syndrome, occurs in approximately one among 3000 to 6000 births. The incidence rate increases as the mother's age increases. Signs and Symptoms: • Growth Deficiency, • Abnormal skull shape and facial features, • Clenched hands, • Rocker bottom feet, • Cardiac and renal abnormalities, • Horse shoe-shaped kidney, • Low set and deformed ears, • Prominent external genitalia, • Small placenta, • Mental retardation, • Hypotonia, • Microcephaly, • Micronagthia (abnormally small jaw), • Cleft lip, • Respiratory Failure -Apnea The survival rate of Edward’s Syndrome is very low, resulting from heart abnormalities, kidney malformations, and other internal organ disorders. About 95% die in utero. Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life. Major causes of death include apnea and heart abnormalities.
Aim: To verify Mendelian laws and study deviations from laws.
Observation: The observed ratio was 30:2.
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of duplicate genes, we hypothesize that the above case represents duplicate genes.
Theory: Gene duplication (or chromosomal duplication) is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination or duplication of an entire chromosome. The second copy of the gene is often free from mutations . So it will have no deleterious effects to its host organism. The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a different function and/or structure. Gene duplication plays a major role in evolution. Plants are the most prolific genome duplicators. For example, wheat is hexaploid (a kind of polyploid), meaning that it has six copies of its genome. There can be complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene.
e.g. Petal color in snapdragon plant.
P: AABB aabb
(Red colour) (white colour)
F1: AaBb
AaBb AaBb
One allele is sufficient to produce the pigment. Whenever a dominant gene is present, the trait is expressed. The phenotypic ratio observed in F2 generation in this case is 15:1.
Discussion: Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Since, the chi square vale is zero, we accept the abovementioned hypothesis. CRI DU CHAT SYNDROME
Cri du chat syndrome or Lejeune’s syndrome, was first described by Jérôme Lejeune in 1963. It is also called as 5p minus or 5p deletion syndrome. It is a group of symptoms that result from missing a piece of chromosome number 5. The syndrome’s name is based on the infant’s cry, which is high-pitched and sounds like a cat.
Genetics:
Cri du chat syndrome (CdCS) is due to a partial deletion of the short arm of chromosome number 5. Approximately 80% of cases results from a sporadic de novo deletion, while about 10-15% are due to unequal segregation of a parental balanced translocation, where the 5p monosomy is often accompanied by a trisomic portion of the genome. The phenotypes in these individuals may be more severe than in those with isolated monosomy of 5p because of this additional trisomic portion of the genome. Most cases involve terminal deletions with 30-60% loss of 5p material. Fewer than 10% of cases have other rare cytogenetic aberrations (eg, interstitial deletions, mosaicisms, rings and de novo translocations). The deleted chromosome 5 is paternal in origin in about 80% of the cases.
Karyotype: 46, XY, del [5p region]
46, XX, del [5p region]
Incidence Rate: 1 in 20,000 to 50,000 live births
Signs and symptoms:
➢ feeding problems because of difficulty swallowing and sucking ➢ low birth weight and poor growth ➢ severe cognitive, speech, and motor delays ➢ widely set eyes ➢ partial webbing or fusing of fingers or toes ➢ behavioral problems such as hyperactivity, aggression, tantrums ➢ unusual facial features which may change over time ➢ excessive dribbling ➢ constipation ➢ microcephaly(small head) ➢ growth retardation ➢ a round face with full cheeks ➢ low birth weight ➢ hypotonia ➢ epicanthal folds (folds of skin above eyes) ➢ down-slanting palpebral fissures ➢ flat nasal bridge ➢ down-turned mouth and high palate ➢ micrognathia(small jaw) ➢ low-set ears ➢ short fingers ➢ single palmar creases(simian crease) ➢ and cardiac defects(ventricular and atrial septal defects)
The Cri du chat affected people are fertile and can reproduce.
Less frequently encountered findings include:
➢ cleft lip and palate ➢ gut malrotation ➢ inguinal hernia ➢ dislocated hips ➢ cryptorchidism(undescended testis) ➢ rare renal malformations (eg horseshoe kidneys)
Late childhood and adolescence findings include:
➢ severe mental retardation ➢ microcephaly ➢ coarsening of facial features ➢ prominent supraorbital ridges ➢ deep-set eyes ➢ single line on the palm of hand (simian crease) ➢ Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time ➢ In males, testes are often small, but spermatogenesis is thought to be normal.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: 9 red seeds; 6 brown seeds
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of complementary genes, we hypothesize that the above case represents complementary genes. This case represents a deviation from Mendelian laws. Theory: Complementary Genes are those nonallelic genes, which independently show a similar effect but produce a new trait when present together in dominant form. Complementary genes were first studied by Bateson and Punnet (1906) in case of flower color of Sweet Pea (Lathyrus odoratus). Here, the flower color is purple if dominant alleles of two genes are present together (C-P-). The color is white if the double dominant condition is absent (ccP-, c-PP, ccpp ). If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure line, homozygous recessive plant with white flowers, the F1 plant will have colored flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but epistatic interactions of the C and P genes will give a modified 9:7 ratio. The following table describes the interactions for each genotype and how the ratio occurs. Genotype Flower Color Enzyme Activities 9 C_P_ Flowers colored; anthocyanin produced Functional enzymes from both genes 3 C_pp Flowers white; no anthocyanin produced p enzyme non-functional 3 ccP_ Flowers white; no anthocyanin produced c enzyme non-functional 1 ccpp Flowers white; no anthocyanin produced c and p enzymes non-functional
It is believed that the dominant gene C produces an enzyme which converts the raw material into chromatogen. The dominant gene P gives rise to an oxidase enzyme that changes chromatogen into purple anthocyanin pigment. This is confirmed by mixing the extract of the two types of flowerts when purple color is formed. Thus purple color formation is two step reaction and the two genes cooperate to form the ultimate product.
Raw material A Chromagen Anthocyanin
Gene C enz Gene P enz
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Calculated chi square value is 0.07. Degrees of freedom =n-1= 2-1 =1 Chi square value from the table is 3.84
0.07<3.84
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis. DOWN’S SYNDROME
Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is the best known and most common chromosome-related disease syndrome formerly known as “mongolism”. It is named after Langdon Down, who first described its clinical signs in 1866.
Genetics:
About 95% of the cases are caused by non-disjunction, with most of the remainder being caused by chromosome translocation. The extra chromosome is contributed by the mother in 90-95% cases. About 75% of these maternal non-disjunctions occur during meiosis I, with the remainder occurring during meiosis II. There is a strong correlation between maternal age and the risk of producing a child with Down’s syndrome. Some patients with Down’s syndrome have a total of 46 chromosomes instead of 47, but in such cases a translocation has joined a part of long arm of chromosome 21 with the long arm of chromosome 14 (14q21q). Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic Down syndrome. This can occur in one of two ways: • a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21. • a Down’s syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
Karyotype: 47,XY,+21
47,XX,+21
46,XX/47,XX,+21 (Mosaic condition)
Incidence Rate: Down’s syndrome is the most common autosomal aneuploidy seen among live births. It is seen in approximately 1/700 live birth, making it the most common aneuploidy condition compatible with survival to term. The risk for mothers less than 25 years of age to have the trisomy is about 1/1500; at 40 years of age, 1/1000; at 45, 1/400. Pregnant women over 45 are a special high-risk group.
Signs and Symptoms:
• Distinct facial features, such as a flat face, small ears, slanting eyes, and a small mouth. • A short neck and short arms and legs. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below-average intelligence. • Occipital is flat • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is a highly consistent feature that is helpful in making a diagnosis. • Fertility amongst both males and females is reduced. Approx 75% of trisomy 21 conceptions are spontaneously aborted, the Down’s syndrome female’s risk of producing affected live-born offspring is considerably lower than 50%. • Heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Most of these problems can be treated.
XYY SYNDROME
XYY syndrome or Jacob's syndrome is a rare chromosomal disorder that affects males. It is caused by an aneuploidy (trisomy) of the sex chromosomes, Y chromosome, thus a human male receives an extra Y chromosome in each cell. The first published report of a man with a 47,XYY Karyotype was by Avery A. Sandberg and colleagues at Roswell Park Memorial Institute in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence that was karyotyped because he had a daughter with Down syndrome.
Genetics:
Males normally have one X and one Y chromosome. However, individuals with Jacob's syndrome have one X and two Y chromosome. Males with Jacob's syndrome, also called XYY males. 47, XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in chromosome separation during metaphase I or metaphase II called no disjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells.
In some cases, the addition of an extra Y chromosome results from non disjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics
Karyotype: 47, XYY.
46, XY/47, XYY mosaics
Incidence Rate: About 1 in 1,000 boys are born with a 47,XYY Karyotype. The incidence of 47,XYY is not affected by advanced paternal or maternal age. Signs and Symptoms: Physical traits: Most often, the extra Y chromosome causes no unusual physical features or medical problems. • 47, XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height. • Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47, XYY. Testosterone levels (prenatally and postnatally) are normal in 47, XYY males. Most 47, XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason. Behavioral traits: • 47, XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. • As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings • Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47, XYY and are managed no differently than in 46,XY males. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
TRIPLE X SYNDROME
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The first published report of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland, in 1959.
Genetics: The condition is also known as triplo-X, trisomy X, XXX syndrome, and 47,XXX aneuploidy. Unlike other chromosonal conditions (such as fragile X), there is usually no distinguishable difference between women with triple X and the rest of the female population. Triple X syndrome is usually not inherited, but occurs as a random event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the nondisjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of her cells. In some cases, trisomy X occurs during cell division in early embryonic development. The additional X chromosome can come from either the maternal or paternal side. The condition is verified only by karyotype testing as it may not be distinguishable phenotypically. • Full trisomy: In this case, trisomy X occurs during gamete formation. • Mosaic trisomy: Some females with triple X syndrome have an extra X chromosome in only some of their cells. In this case, trisomy X occurs during embryogenesis.
Karyotype:
• Full trisomy: 47,XXX • Mosaic: 46,XX/47,XXX Incidence Rate: Triple X syndrome occurs in around 1 in 1,000 live female births. Signs and symptoms: • Due to inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell and two Barr bodies are visible in somatic cell . Thus, triple X syndrome most often causes no unusual physical features or medical problems. • Females with the condition may have menstrual irregularities. • Although they rarely exhibit severe mental impairments, they have an increased risk of learning disabilities, delayed speech, and language skills. • a lanky/youthful appearance with increased facial beauty has been described, or in some instances varying degrees of androgeny, but these cases usually reflect traits present in near relatives. • Most women with triple X have normal sexual development and are able to conceive children. • A few may experience an early onset of menstruation. • Early menopause. • Triple X women are rarely diagnosed, apart from pre-natal testing methods, such as amniocentesis. Most medical professionals do not regard the condition a disability. However, such status can be sought by parents for early intervention treatment if mild delays are present.
TURNER’S SYNDROME
The syndrome is named after Henry Turner, an Oklahoma endocrinologist, who described it in 1938. The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London. It was found in a 14-year-old girl with signs of Turner syndrome.
Genetics: Turner’s syndrome encompasses several conditions, of which monosomy XO is the most common. Instead of the normal XX sex chromosomes for a female, (or XY for a normal male) only one X chromosome is present and fully functional; in rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). In Turner’ssyndrome, female sexual characteristics are present but generally underdeveloped. The risk factors for Turner’s syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome. There is currently no known cause for Turner syndrome, though there are several theories surrounding the subject. Karyotype: • Full Monosomy: 45,XO • Mosaic Monosomy: 46,XX/45,XO Incidence Rate: Approximately 98% of all fetuses with Turner syndrome result in miscarriage. Turner syndrome accounts for about 10% of the total number of spontaneous abortions in the United States. The incidence of Turner syndrome in live female births is believed to be 1 in 2500. Signs and Symptoms: -Short stature -Lymphedema (swelling) of the hands and feet -Broad chest , poor breast development and widely-spaced nipples -Low hairline -Low-set ears, hearing loss -Reproductive sterility, rudimentary ovaries, gonadal streak (underdeveloped gonadal structures) -Amenorrhea or the absence of a menstrual period -Increased weight, obesity -Shortened metacarpal IV (of hand) -Characteristic facial features,visual impairments -Webbing of the neck (webbed neck) -Congenital heart disease-Coarctation of the aorta -Horseshoe kidney -Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. -Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. -Approximately one-third of all women with Turner syndrome have a thyroid disorder. -Moderately increased risk of developing diabetes. -Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. -Women with Turner syndrome are almost universally infertile. Even when pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner’s Syndrome or Down’s Syndrome. -Other symptoms may include a small lower jaw (micrognathia), cubitus valgus (turned-out elbows), soft upturned nails, palmar crease and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition, and no two individuals will share the same symptoms.
AIM: To prove monogenic inheritance using the result of PTC (phenyl thiocarbamide) test, thus confirming the Hardy Weinberg’s Law. REQUIREMENTS: Beaker, PTC, alcohol, strips of Whatmann’s paper, oven. THEORY: This law was proposed by G.H. Hardy and W.Weinberg independently in 1908. They reported that under certain conditions hereditary conservation of genes is the characteristic of population. This is a static condition and the population is non evolving. Such a kind of stability at the genetic level is called genetic equilibrium. Hardy Weinberg Law: the relative frequency of alleles in the population remains constant from generation to generation in a population of sexually reproducing organisms when- • The population is large enough so that random sampling errors do not affect the allele frequency. • Mating takes place at random. • Mutation does not take place or if it does the rate is same in both directions • All members of population survive and have equal reproductive rates. • There should be no migration among different populations. In reality no population satisfies Hardy Weinberg’s Law. But in large natural populations with little migration and negligible natural selection, the Hardy Weinberg Law may be nearly approximated. Deviations from Hardy Weinberg Law: Mutations: they produce alternate alleles at a given locus and alter the phenotype. In a population, mutation disturbs the genetic equilibrium and gene frequency. Thus gene pool gradually changes with mutant gene appearing with greater frequency. Selection: if a mutation is advantageous then natural selection may preserve it and such characters get inherited and contribute more to the gene pool of next generation. Thus genetic equilibrium is changed. e.g. heterozygous advantage in case of sickle cell anemia. Non random mating: Differential mating is one of the evolutionary forces resulting in abundance of certain genotype at the cost of others. Small population size and genetic drift: the random change in gene frequency occurring by chance and not under the control of natural selection is a genetic drift. In populations genetic drift favors either a loss or fixation of an allele. The rate at which an allele is lost or becomes fixed, depends on the population size.
Significance of the Hardy Weinberg Law: It provides a tool which helps the population geneticists to determine the degree of evolutionary change by comparing allele frequencies at starting point and at some future point. By this law we can calculate the frequencies of homozygous dominant, homozygous recessive and heterozygous carriers in a population.
PROCEDURE: 1. In a beaker, 1% solution of PTC was prepared in 100% alcohol. 2. Several small strips of Whatman’s paper were dipped in at least for 2-3 hours. 3. The strips were allowed to dry in an oven for about half an hour. 4. After drying, the strips were tasted. OBSERVATION:
No. of positive tasters No. of non tasters population size 17 2 19
CALCULATION: According to Hardy Weinberg Law :
p2 + q2 + 2pq = 1
Where, p = allelic frequency of dominant
q = allelic frequency of recessive
2pq = heterozygous dominant frequency
p2 = homozygous dominant frequency
q2 = homozygous recessive frequency
Phenotypic frequency for the population-
Dominant = P2 + 2pq = 17/19 = 0.894
Recessive = q2 = 2/19 = 0.106
Therefore, q = 0.325
And as we know that p+q = 1 ;
P = 1- q = 0.675
Allelic frequency-
Dominant: p = 0.675
Recessive: q = 0.325
Genotypic frequency for population:
Homozygous dominant: p2 = (0.675)2 = 0.45
Homozygous recessive: q2 = (0.325)2 = 0.10
Heterozygous dominant: 2pq = 0.44
RESULT: Monogenetic inheritance in man was proved using result of PTC, thus confirming HARDY-WEINBERG’S LAW
PRECAUTIONS: 1. Whatman’s strips should be dipped in PTC solution for at least 2-3 hours. 2. The strips should always be properly dried before use. 3. Results should be accurately observed. 4. PTC solution was finely prepared
Aim : To calculate linkage number .
Problem 1:
A cross is made between homozygous wild-type female Drosophila (a+a+b+b+ c+c+) and triple-mutant males (aa bb cc) (the order here is arbitrary). Give the constitution of gametes produced by the parents and also genotype of F1 . A test cross of the F1 gave the following phenotypes:
“a+ b c” 18 “a b+ c” 112 “a b c” 308 “a+ b+ c” 66 “a b c+” 59 “a+ b+ c+” 321 “a+ b c+” 102 “a b+ c+” 15
1000
1. Calculate the percentage of crossover between 3 gene pairs 2. Calculate the percentage of double crossover and coefficient of coincidence 3. Make a linkage map of these genes
Problem 2:
A cross was made between C/C , sh/sh , Wx/Wx stocks of maize and one with the genotype c/c , Sh/Sh , wx/wx. Give the constitution of the gametes produced by theparents and the genotype of F1 A test cross of F1 gave the following phenotype :
C sh Wx 2777 c Sh wx 2708 C Sh wx 116 c sh Wx 122 C sh wx 643 c Sh Wx 626 C Sh Wx 4 c sh wx 3 TOTAL 7000
1) Calculate the percentage of crossover between 3 gene pairs 2) Calculate the percentage of double crossover and coefficient of coincidence 3) Make a linkage map of these genes
PENETRANCE
• It is the frequency with which a heritable trait is manifested by individuals carrying the principal gene or genes conditioning it.
• In other words, the percentage of individuals that show at leastr some degree of expression of a mutant genotype defines the penetrance.
• The presence of a gene may not result in a detectable phenotype.
Example:-
The phenotypic expression of many mutant alleles in drosphila is indistinguishable from wild type.If 15 percent of mutant flies show the wild type appearance ,the mutant gene is said to have a penetrance of 85 percent.
VARIABLE EXPRESSIVITY
• Expressivity reflects the range of expression of the mutant genotype. • In other words it is the variation in the degree to which gene is expressed. • the variability with which basic patterns of inheritance are modified, both in degree and in variety, by the effect of a given gene in people of the same genotype.
Example:-
polydactyly may be expressed as extra toes in one generation and extra fingers in another.
AIM : To Study Polyploidy In Allium cepa Root Tip
Things Required : Slide, Colchicine, 1N HCl, Acetocarmine Dye, Onion Root Tip, needle, forceps, Theory : Polyploidy occurs in cells and organisms when there are more than two homologous sets of chromosomes. Polyploidy is a state different from most organisms which are normally diploid meaning they have only two sets of chromosomes - one set inherited from each parent; polyploidy may occur due to abnormal cell division. It is most commonly found in plants.
Haploidy may also occur as a normal stage in an organism's life. A haploid has only one set of chromosomes. Polyploidy occurs in some animals, such as goldfish, salmon, and salamanders, but is especially common among ferns and flowering plants (see Hibiscus Rosa-Sinensis), including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat. Many agriculturally important plants of the genus Brassica are also tetraploids; their relationship is described by the Triangle of U. The occurrence of polyploidy is a mechanism of speciation and is known to have resulted in new species of the plant Salsify (also known as "goatsbeard").
Speciation via polyploidy: A diploid cell undergoes failed meiosis, producing diploid gametes, which self-fertilize to produce a tetraploid zygote. Polyploidy can be induced in cell culture by some chemicals: the best known is colchicines, which can result in chromosome doubling, though its use may have other less obvious consequences as well.
Polyploid types are labelled according to the number of chromosome sets in the nucleus: • triploid (three sets; 3x), for example the phylum Tardigrada • tetraploid (four sets; 4x), for example Salmonidae fish • pentaploid (five sets; 5x) • hexaploid (six sets; 6x), for example wheat, kiwifruit • oktoploid (eight sets; 8x), for example Acipenser (genus of sturgeon fish) • dekaploid (ten sets; 10x), for example certain strawberries • dodecaploid (twelve sets; 12x), for example the plant Celosia argentea
Autopolyploidy Autopolyploids are polyploids with multiple chromosome sets derived from a single species. Autopolyploids can arise from a spontaneous, naturally occurring genome doubling (for example, the potato). Others might form following fusion of 2n gametes (unreduced gametes). Bananas and apples can be found as triploid autopolyploids. Autopolyploid plants typically display polysomic inheritance, and are therefore often infertile and propagated clonally Allopolyploidy Allopolyploids are polyploids with chromosomes derived from different species. Triticale is an example of an allopolyploid, having six chromosome sets, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid. Some of the best examples of allopolyploids come from the Brassicas, and the Triangle of U describes the relationships among the three common diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus, B. juncea, and B. carinata) derived from hybridization among the diploids. Polyploidy in plants Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes Huge explosions in angiosperm species diversity appear to have coincided with the timing of ancient genome duplications shared by many species. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (eg. potato) and allopolyploids (eg. canola, wheat, cotton) can be found among both wild and domesticated plant species. Most polyploids display heterosis relative to their parental species, and may display novel variation or morphologies that may contribute to the processes of speciation and eco-niche exploitation The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and/or expression levels. Many of these rapid changes may contribute to reproductive isolation and speciation. There are few naturally occurring polyploid conifers. One example is the giant tree Sequoia sempervirens or Coast Redwood which is a hexaploid (6x) with 66 chromosomes (2n=6x=66), although the origin is unclear. Polyploid crops Polyploid plants tend to be larger and better at flourishing in early succession habitats such as farm fields in the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops (and agricultural weeds) may have unintentionally been bred to a higher level of ploidy. The induction of polyploidy is a common technique to overcome the sterility of a hybrid species during plant breeding. For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids are sterile. After polyploidization, the hybrid becomes fertile and can thus be further propagated to become triticale. In some situations polyploid crops are preferred because they are sterile. For example many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine. Examples of Polyploid Crops • Triploid crops: banana, apple, ginger, watermelon, citrus • Tetraploid crops: durum or macaroni wheat, maize, cotton, potato, cabbage, leek • Hexaploid crops: chrysanthemum, bread wheat, triticale, oat, kiwifruit • Octaploid crops: strawberry, dahlia, pansies, sugar cane Polyploidy in animals Examples in animals are more common in the 'lower' forms such as flatworms, leeches, and brine shrimp. Polyploid animals are often sterile, so they often reproduce by parthenogenesis. Polyploid salamanders and lizards are also quite common and parthenogenetic. While mammalian liver cells are polyploid, rare instances of polyploid mammals are known, but most often result in prenatal death. One of the only known exceptions to this 'rule' is an octodontid rodent of Argentina's harsh desert regions, known as the Red Viscacha-Rat (Tympanoctomys barrerae). This rodent is not a rat, but kin to guinea pigs and chinchillas. Its "new" diploid [2n] number is 102 and so its cells are roughly twice normal size. Its closest living relation is Octomys mimax, the Andean Viscacha-Rat of the same family, whose 2n=56. It is surmised that an Octomys-like ancestor produced tetraploid (i.e., 4n=112) offspring that were, by virtue of their doubled chromosomes, reproductively isolated from their parents; but that these likely survived the ordinarily catastrophic effects of polyploidy in mammals by shedding (via translocation or some similar mechanism) the "extra" set of sex chromosomes gained at this doubling.. Polyploidy in humans (Aneuploidy) True polyploidy rarely occurs in humans, although it occurs in some tissues (especially in the liver).
Polyploidy refers to a numerical change in a whole set of chromosomes. Organisms in which a particular chromosome, or chromosome segment, is under- or overrepresented are said to be aneuploid (from the Greek words meaning "not," "good," and "fold"). Therefore the distinction between aneuploidy and polyploidy is that aneuploidy refers to a numerical change in part of the chromosome, whereas polyploidy refers to a numerical change in the whole set of chromosomes.
Polyploidy occurs in humans in the form of triploidy (69,XXX) and tetraploidy (92,XXXX), not to be confused with 47,XXX or 48, XXXX aneuploidy. Triploidy, usually due to polyspermy, occurs in about 2-3% of all human pregnancies and ~15% of miscarriages. The vast majority of triploid conceptions end as miscarriage and those that do survive to term typically die shortly after birth. In some cases survival past birth may occur longer if there is mixoploidy with both a diploid and a triploid cell population present. Triploidy may be the result of either digyny (the extra haploid set is from the mother) or diandry (the extra haploid set is from the father). Diandry is almost always caused by the fertilization of an egg by two sperm (dispermy). Digyny is most commonly caused by either failure of one meiotic division during oogenesis leading to a diploid oocyte or failure to extrude one polar body from the oocyte. Diandry appears to predominate among early miscarriages while digyny predominates among triploidy that survives into the fetal period. However, among early miscarriages, digyny is also more common in those cases <8.5 weeks gestational age or those in which an embryo is present. There are also two distinct phenotypes in triploid placentas and fetuses that are dependent on the origin of the extra haploid set. In digyny there is typically an asymmetric poorly grown fetus, with marked adrenal hypoplasia and a very small placenta. In diandry, the fetus (when present) is typically normally grown or symmetrically growth restricted, with normal adrenal glands and an abnormally large cystic placenta that is called a partial hydatidiform mole. These parent-of-origin effects reflect the effects of genomic imprinting. Complete tetraploidy is more rarely diagnosed than triploidy, but is observed in 1-2% of early miscarriages. However, some tetraploid cells are commonly found in chromosome analysis at prenatal diagnosis and these are generally considered 'harmless'. It is not clear whether these tetraploid cells simply tend to arise during in vitro cell culture or whether they are also present in placental cells in vivo. There are, at any rate, very few clinical reports of fetuses/infants diagnosed with tetraploidy mosaicism. Mixoploidy is quite commonly observed in human preimplantation embryos and includes haploid/diploid as well as diploid/tetraploid mixed cell populations. It is unknown whether these embryos fail to implant and are therefore rarely detected in ongoing pregnancies or if there is simply a selective process favoring the diploid cells. Procedure : 1. Keep the onion root tip in 1% colchicine for 10 -24 hrs at 40 C or for 3-6 hrs at room temperature 2. Transfer in Distilled water 3. Then keep the sample in 1N HCl for 10 minutes 4. wash with water 5. excise the root tip portion 6. add few drops of acetocarmine dye and keep for 15 minutes 7. Tease with needle 8. Heat fix by passing throgh the flame 2 – 3 times 9. Cover with cover slip and tap and view under 100x
Observations: 23 chromosomes observed instead of 16, confirming polyploidy.
Precautions: 1. Slide should not be over heated 2. Stain should not be used in excess 3. Tapping should be done gently.
Aim:
To study polytene chromosome using permanent slides.
Theory:
Polytene chromosomes were originally observed in the larval salivary glands of Chironomus midges by Balbiani in 1881. The hereditary nature of these structures was given by Emil Heitz and Hans Bauer in early 1930s by their studies on Drosophila melanogaster.
To increase cell volume, some specialized cells undergo repeated rounds of DNA replication without cell division (endomitosis), forming a giant polytene chromosome. Polyteny is achieved by replication of the DNA several times without nuclear division and the resulting daughter chromatids do not separate and remain aligned side by side. A polytene chromosome in the cells of Drosophila salivary glands has about a thousand DNA molecules arranged side by side, all attached at their centromeres, which arise from ten rounds of DNA replication.
Polytene chromosomes occur in salivary gland, trachea, fat cells and malphigian tubules of many insects. They are known to occur in secretary tissues of other dipteran insects such as the Malpighian tubules of Sciara and also in protists, plants, mammals, or in cells from other insects. Some of the largest polytene chromosomes known occur in larval salivary gland cells of the Chironomid genus Axarus.
In polytene cells the chromosomes are visible during interphase. Polytene chromosomes have characteristic light and dark banding patterns. Dark banding frequently corresponds to inactive chromatin, while light banding is usually found at areas with higher transcriptional activity. The chromomeres (regions in which chromatin is more tightly coiled) alternate with regions where the DNA fibers are loosely folded. The banding patterns of the polytene chromosomes of Drosophila melanogaster were sketched in 1935 by Calvin B. Bridges. The banding patterns of the chromosomes are especially helpful in research, as they provide an excellent visualization of transcriptionally active chromatin and general chromatin structure. The banding pattern is specific for each pair of homologous chromosomes. They have their own characteristic morphology and position which permits detailed chromosome mapping. The polytene chromosomes consist of coiled or associated homologous pairs of chromosomes. This sort of association is termed as somatic pairing. This permits the identification of abnormalities like deletion, inversions and duplications as regions looped out of the chromosomes.
In addition to increasing the volume of the cell's nuclei and causing cell expansion, polytene cells may also have a metabolic advantage as multiple copies of genes permits a high level of gene expression. In Drosophila melanogaster, for example, the chromosomes of the larval salivary glands undergo many rounds of endoreplication, to produce large amounts of glue before pupation.
The polytene chromosomes develop swellings at particular points. Such chromosomal swellings are called Chromosome Puffs. They are diffuse uncoiled regions of the polytene chromosome that are sites of active RNA transcription and commonly occur in bands. In polytene chromosomes a series of loops may be given out laterally. These loops are called Balbiani Rings, which are large chromosome puffs. They are rich in DNA and mRNA.
Observation:
1. Polytene chromosomes are giant chromosomes with cable like structure. The maternal and paternal homologues remain associated side by side in somatic pairing which have undergone multiple DNA replications.
2. They show characteristic morphology in which dark bands alternate with interbands.
3. The chromosomes also contained swellings or puffs at particular points (mainly in dark band region). These swelling are called Balbiani Rings.
SEX LINKED INHERITANCE
Y-Linked inheritance ( Hollandric Inheritance) :
• Y-linked inheritance occurs when a gene, trait, or disorder is transferred through the Y chromosome. • Since Y chromosomes can only be found in males, Y linked traits are only passed on from father to son. • The testis determining factor, which is located on the Y chromosome, determines the maleness of individuals. Besides the maleness inherited in the Y-chromosome there are no other found Y-linked characteristics. • It is passed from father to all sons i.e. 100% male progeny will be affected. • It does not skip generations.
X-Linked Dominant Inheritance :
• Both males and females are affected ; often more females than males are affected • Does not skip generations. Affected sons must have an affected mother: affected daughters must have either an affected mother or an affected father. • Affected fathers will pass the trait on to all their daughters. • Affected mothers (if heterozygous ) will pass the trait on to ½ of their sons and ½ of their daughters. • An example of an X-linked dominant trait in humans is hypophosphatemia or familial vitamin D-resistant rickets. People with this trait have features that superficially resemble those produced by rickets.
X-Linked Recessive Inheritance:
• More males than females are affected. • Affected sons are usually born to unaffected mothers; thus, the trait skips generations. • Approximately ½ of a carrier (heterozygous) mother’s sons are affected. • It is never passed from father to son. • All daughters of affected fathers are carriers. • An example of an X-linked recessive trait in humans is color blindness.
DOWN’S SYNDROME Down’s syndrome is a form of chromosomal variation or aneuploidy characterized by the presence of an extra chromosome 21 in body. Children with Down’s syndrome tend to have certain features, such as a flat face and a short neck. They also have some degree of mental retardation. This varies from person to person, but in most cases it is mild to moderate. Down syndrome is a lifelong condition. But with care and support, most children with down syndrome can grow up to have healthy, happy, productive lives. It is named after Langdon Down, who first described its clinical signs in 1866 and formerly known as “Mongolism”. GENOTYPE Male - Full/partial (mosaic) Female -Disorder was identified as chromosome 21 trisomy by Jerome Lejuene in 1959. INCIDENCE RATE The incidence of Down syndrome is estimated at one per 800 to one per 1000 births. The incidence rate increases with increase in maternal age. CAUSES • Maternal age: influences the chances of conceiving a baby with Down syndrome. At maternal age 20 to 24 , the probability is one in 1562; at age 35 to 39 the probability is one in 214, and above age 45 the probability is one in 19. Although the probability increases with maternal age, 80% of children with Down syndrome are born to women under the age of 35, reflecting the overall fertility of that age group. About 75% of these maternal non –disjunctions occur during meiosis I, with the reminder occurring during meiosis II. Recent data also suggest that paternal age, especially beyond 42, also increases the risk of down syndrome manifesting in pregnancies in older mothers. • NON-DISJUNCTION: Non-Disjunction event during gametogenesis or early cell division during embryogenesis may result in full or mosaic Down’s syndrome respectively. Out of 95% of total cases, 88% causes result due to non-disjunction event during female gametogenesis and 8% during gametogenesis in males.
• MOSAICISM: Trisomy 21 is usually caused by non-disjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic. Down syndrome (46, XX/47, XX, +21). This can occur in one of two ways
1. A non-disjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21.
2. Down syndrome embryo undergoes non-disjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the causes of 1-2% of the observed Down syndrome
SYMPTOMS:
Most children with Down syndrome have:
• Distinct facial features, such as a flat face, small ears, flat nasal bridge, slanting eyes, oblique parperal fiscers and a small mouth with protruding tongue. • A short neck and short arms and legs. • Excessive space between large toe and second toe. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below average intelligence. • Occipital is flat. • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is highly consistent feature that is helpful in making a diagnosis.
Many children with Down syndrome are also born with heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Luckily, most of these problems can be treated
PATAU’S SYNDROME
Patau’s syndrome, also called trisomy 13, is a congenital disorder associated with the presence of an extra copy of chromosome 13. The extra chromosome 13 causes numerous physical and mental abnormalities, especially heart defects. Patau syndrome is named after Dr. Klaus Patau, who reported the syndrome and its association with trisomy in 1960.
Genetics:
75 to 80% of the cases of Patau syndrome are caused by trisomy of chromosome 13. Some of these cases are the result of a total trisomy, while others are the result of a partial trisomy. Partial trisomy generally causes less severe physical symptoms than full trisomy. 10% of these cases are of the mosaic type, in which only some of the body’s cells have the extra chromosome. The physical symptoms of the mosaic form of Patau syndrome depends on the number and type of cells that carry the trisomy. Most of the cases of trisomy are not passed on from one generation to the next. Usually they result from a malfunction in the cell division (mitosis) that occurs after conception. At least 75% of the cases of Patau syndrome are caused by errors in chromosome replication that occur after conception. The remaining 25% are caused by the inheritance of translocations of chromosome 13 with other chromosomes within the parental chromosomes. In these cases, a portion of another chromosome switches places with a portion of chromosome 13. This leads to errors in the genes on both chromosome 13 and the chromosome from which the translocated portion originated.
Karyotype:
Full trisomy: 47,XX,+13
47,XY,+13
Mosaic trisomy: 47,XX/46,XX,+13
Incidence Rate:
Incidence rate is 1 in 6000 births, births may be falling due to prenatal screening and selective termination of pregnancy. The risk of Patau syndrome seems to increase with the mother’s age, particularly if she is over 30 when pregnant. Male and female children are equally affected, and the syndrome occurs in all races.
Signs and symptoms:
• Mental and motor retardation. • Polidactyly (extra digits) • Microcephaly • Low set ears • Holoprospncephaly (failure of the fore brain two divide properly) • Heart defects (80% of the cases) • Structural eye defects, including Micropthalmia, retinal Dysplasia or retinal detachment, cortical visual loss and optic nerve hypoplasia • Cleft palate or hare lip (not complete fusion of soft and hard palate) • Meningomyelocete (spinal defect or motor abnormality) • Omphalocete (abdominal defect) • Abnormal Genitalia • Abnormal Palm pattern • Overlapping of fingers over thumb • Micrognathia (small jaws) • Hypertonia (low muscle tone) • Cryptorchoidism
The life expectancy is very limited. Most die in the first week and few survive beyond a year. The median age of death is 2.5 days. More than 80% die within a month. Only 5 % last 6 months. The maximum life Span is 21 years. In many cases, spontaneous abortion (miscarriage) occurs and the fetus does not survive to term. In other cases, the affected individual is stillborn.
PHILADELPHIA CHROMOSOME
Philadelphia chromosome is referred to an abnormally short chromosome 22, one of the two chromosomes involved in translocation with chromosome 9 in this condition. This chromosome abnormality causes chronic myeloid leukemia (CML) and is also seen in some cases of acute lymphoblastic leukemia (AML). The Philadelphia chromosome was first discovered and described in 1960 by Peter Nowell from University of Pennsylvania School of medicine and David Hungerford from the Fox Chase Cancer Center’s Institute for cancer research and was therefore named after the city in which both centers were located. The mechanism by which the Philadelphia chromosome arises as a result of translocation was however identified by Janet D. Rowley at the University of Chicago in 1973.
Molecular Genetics:
Due to translocation, a part of bcr (Breakpoint Cluster Region) gene from chromosome 22 gets fused with part of the abl (Abelson) gene on chromosome 9. The abl gene (22, q11) encodes for tyrosine kinase protein in a regulated manner. During reciprocal translocation, parts of these two chromosomes swap pieces, which lead to fusion of abl-bcr on shorter chromosome 22 (Ph chromosome). This fused gene encodes for a 21 kD protein with tyrosine kinase enzyme activity but in uncontrolled amount as regulatory sequences may have been dissociated from gene during translocation. The BCR-ABL protein is expressed at unregulated levels and interacts with interleukin β3 receptor subunit, which in turn activates number of cell cycle controlling proteins thus speeding the cell division. It also results in inhibition of DNA repair, causing genomic instability. The efficacy in CML of a drug that inhibits the BCR-ABL tyrosine kinase has provided the final proof that the BCR-ABL oncoprotein is the unique cause of CML. Nomenclature:
t(9;22)q34;q11) based on International System for Human Cytogenetic Nomenclature.
Incidence Rate:
95% of patients suffering from Chronic Myeloid Leukemia while only 10-12% patients suffering from acute lymphoblast leukemia show this abnormality.
Disease and Symptoms:
Philadelphia chromosome causes Chronic Myeloid Leukemia (CML). This translocation occurs in single bone marrow cell and through process of clonal expansion, gives rise to leukemia. A progressive blood and bone marrow disease usually occurs during or after middle age and rarely in children. Normally the bone marrow makes blood stem cells that develop into mature blood cells after some time. A blood stem cell may become a myeloid stem cell or a lymphoid cell that develops into white blood cell (WBC). In CML, too many blood stem cells develop into WBCs. These WBCs are abnormal and do not develop into healthy WBC’s. These cells build up in bone marrow leaving less room for healthy cells resulting in anemia.
Symptoms:
• Mild to moderate anaemia • Fatigue • Low grade fevers or sweats • Fullness in the abdomen caused by enlarged spleen • left-upper-quadrant pain caused by splenic infarction
Aim: To verify Mendelian law and study deviations from laws.
Observation: The observed ratio of different variety of seeds was 22:7.
Result: Since the observed ratio is close to the phenotypic ratio observed in case of a monohybrid cross, we hypothesize that the above case represents monohybrid cross.
Theory: A Monohybrid cross is a cross between parents who are heterozygous at one locus. It is a breeding experiment dealing with a single character. For example, a monohybrid cross between two pure-breeding plants (homozygous for their respective traits), one with yellow seeds (the dominant trait) and one with green seeds (the recessive trait), would be expected to produce an F1 (first) generation with only yellow seeds because the allele for yellow seeds is dominant to that of green. A monohybrid cross compares only one trait. In this example, both organisms have the genotype Bb. They can produce gametes that contain either the B or b alleles. The probability of an individual offspring having the genotype BB is 25%, Bb is 50%, and bb is 25%. Maternal B b Paternal B BB Bb b Bb bb It is important to note that Punnett squares only give probabilities for genotypes, not phenotypes. The way in which the B and b alleles interact with each other to affect the appearance of the offspring depends on how the gene products (proteins) interact. For classical dominant/recessive genes, like that which determines whether a rat has black hair (B) or white hair (b), the dominant allele will mask the recessive one. Thus in the example above 75% of the offspring will be black (BB or Bb) while only 25% will be white (bb). The ratio of the phenotypes is 3:1, typical for a monohybrid cross. Discussion: On calculating the chi-square value it was found to be 0.011, when this value was compared with the standard table showing relation between degree of freedom and chi square value for 5% probability it was found to be less. Thus the hypothesis is accepted.
KLINEFELTER’S SYNDROME
Klinefelter’s syndrome is a genetic disorder, which affects males, causing reduced fertility and development of small testicles. Affected individuals have atleast one Y chromosome and atleast two X chromosomes. It is named after Dr. Harry klinefelter , an endocrinologist who first described it in 1942.
Genotype:
It is a condition caused by chromosome nondisjunction in males; affected individuals have a pair of X sex chromosomes instead of just one. About 50-60% of the cases are due to maternal nondisjunction (75% meiosis I errors). Despite the relatively mild phenotypic features of this disorder, it is estimated that at least half of the 47, XXY conceptions are spontaneously aborted. In mammals with more than one X chromosome, the genes on all but one X chromosome are not expressed; this is known as X inactivation. This happens in XXY males as well as normal XX females. A few genes located in the pseudoautosomal regions, however, have corresponding genes on the Y chromosome and are capable of being expressed. These triploid genes in XXY males may be responsible for symptoms associated with Klinefelter's syndrome.
Karyotype: 47,XXY Males
48, XXXY Males
Incidence rate:
The condition is reported in roughly 1 out of every 500 males.
Signs and Symptoms:
• Taller than average individuals with longer arms and legs • Slightly feminized physique with reduced body hair • Small testis (microorchidism). • Serum testosterone levels are lower than normal. • Urinary gonadotropin levels are elevated. • Infertility (azoospermia) may result from atrophy of seminiferous tubules. • Gynecomastia (breast development). • Mild degree of mental retardation. • Language learning impairment but not mentally retarded.
Aim: To study the human karyotype.
Theory:
A karyotype is the characteristic chromosome complement of a eukaryote species.The preparation and study of karyotypes is part of cytogenetics.
In normal diploid organisms, autosomal chromosomes are present in two identical copies. There may, or may not, be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies. The study of whole sets of chromosomes is also known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.The chromosomal pairs are arranged in descending order of size with autosomes followed by sex chromosomes.
The study of karyotypes is made possible by staining: usually a suitable dye is applied after cells have been arrested during cell division by a solution of colchicine. Sometimes observations may be made on non-dividing (interphase) cells. The sex of an unborn foetus can be determined by observation of interphase cells .
Most (but not all) species have a standard karyotype. The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Normal karyotypes for women contain two X chromosomes and are denoted 46,XX; men have both an X and a Y chromosome denoted 46,XY. Any variation from the standard karyotype may lead to developmental abnormalities.
Purpose of Karyotype:
Karyotypes can be used for many purposes:
- to study chromosomal aberrations
- to study cellular function
- to study taxonomic relationships, or
- to gather information about past evolutionary events.
Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; one well suited example is the Philadelphia chromosome, a translocation mutation associated with chronic myelogenous leukemia and less often wuth acute lymphoblastic leukemia.
Observations in a Karyotype:
Six different characteristics of karyotypes are usually observed and compared:
- differences in absolute sizes of chromosomes. Chromosomes can vary in absolute size by as much as twenty-fold between genera of the same family: Lotus tenuis and Vicia faba (legumes), both have six pairs of chromosomes (n=6) yet V. faba chromosomes are many times larger. This feature probably reflects different amounts of DNA duplication.
- differences in the position of centromeres. This is brought about by translocations.
- differences in relative size of chromosomes can only be caused by segmental interchange of unequal lengths.
- differences in basic number of chromosomes may occur due to successive unequal translocations which finally remove all the essential genetic material from a chromosome, permitting its loss without penalty to the organism (the dislocation hypothesis). Humans have one pair fewer chromosomes than the great apes, but the genes have been mostly translocated (added) to other chromosomes.
- differences in number and position of satellites, which (when they occur) are small bodies attached to a chromosome by a thin thread.
- differences in degree and distribution of heterochromatic regions. Heterochromatin stains darker than euchromatin, indicating tighter packing, and mainly consists of genetically inactive repetitive DNA sequences.
A full account of a karyotype may therefore include the number, type, shape and banding of the chromosomes, as well as other cytogenetic information.
After sorting by size, chromosomes are further classified as :
Position of the centromere:
Metacentric- having centromere in the centre.
Submetacentric- having centromere near the centre.
Acrocentric- having centromere near the end.
Telocentric- having centromere at the end.
Number of centromeres:
Monocentric- having a single centromere.
Dicentric-having two centromeres.
Polycentric-having many centromeres.
Holocentric-absence of functional centromere.Generally, a dispersed centromere is present over the entire chromosome.
Variation is often found:
- between the two sexes
- between the germ-line and soma (between gametes and the rest of the body)
- between members of a population (chromosome polymorphism)
- geographical variation between races
- mosaics or otherwise abnormal individuals
Human Chromosome Groups:
Group Chromosomes Description A 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentric B 4,5 Large; submetacentric with two arms very different in size C 6–12,X Medium size; submetacentric D 13–15 Medium size; acrocentric with satellites E 16–18 Small; 16 is metacentric but 17 and 18 are submetacentric F 19,20 Small; metacentric G 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the Y Autosomes are numbered from largest to smallest, except that chromosome 21 is smaller than chromosome 22.
Depiction Of Karyotypes: Cytogenetics employs several techniques to visualize different aspects of chromosomes. G-banding :It is obtained with Giemsa stain following digestion of chromosomes with trypsin.It brings out sulfur rich proteins. It yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich. This method will normally produce 300-400 bands in a normal, human genome. R-banding :It is the reverse of G-banding (the R stands for "reverse"). The dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions).It requires heat treatment so as to denature the bonds between adenine and thymine (and other regions get stained). C-banding: Giemsa binds to constitutive heterochromatin, so it stains centromeres. Q-banding: It is a fluorescent pattern obtained using quinacrine mustard for staining. The pattern of bands observed using fluorescent microscope is very similar to that seen in G-banding. T-banding: visualize telomeres. In the "classic" karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms p and q, respectively. In addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,del(5)(p15.2) Chromosome abnormalities: Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in translocations, inversions, large-scale deletions or duplications. Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete; trisomies, in which three copies of a chromosome are present instead of the usual two, are common numerical abnormalities. Structural abnormalities often arise from errors in homologous recombination. Both types of abnormalities can occur in gametes and therefore will be present in all cells of an affected person's body, or they can occur during mitosis and give rise to a genetic mosaic individual who has some normal and some abnormal cells. Chromosomal abnormalities that lead to disease in humans include Turner syndrome, Klinefelter syndrome, Edwards syndrome, Down syndrome, Patau syndrome, trisomy 8, trisomy 9 and trisomy 16. Some disorders arise from loss of just a piece of one chromosome, including Cri du chat, 1p36 Deletion syndrome, Angelman syndrome. All such abnormalities can be diagnosed using a karyotype.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: Given ratio of seeds 4 Grams colored seeds 8 Black colored seeds 4 Green colored seeds Result:
Since the observed ratio is close to the phenotypic ratio observed in case of incomplete dominance, we hypothesize that the above case represents incomplete dominance. This case represents a deviation from Mendelian laws.
Theory:
Mendel gave the law of inheritance for Monohybrid as well as Dihybrid cross for F1 & F2 Generation. For a monohybrid cross between RR tall character (dominant) and rr dwarf character (recessive), the mendelian ratio of phenotype for F2 Generation is 3:1 (3 tall, 1 dwarf) and genotypic ratio is 1:2:1.
The incomplete dominance was observed in Antirrhinum majus. The RR-red character, rr-white when crossed the flowers in F1 generation were all pink. When F1 generation flower is self crossed the flowers of F2 generation were 1(red), 2 (pink), 1 (white) which shows that 1:2:1 was followed.
RR rr
F1 generation Rr (all pink)
F2 generation R r
RR Rr
Rr Rr
R
r
RR-red Rr-pink rr-white
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance
Expected ratio: 1:2:1 Observed ratio: 1/4*8 =2 1/2*8=4 1/4*8=2 Degree of freedom for given example (n-1) (3-1)=2 X2 test is applied. X2= (obs-exp)2 /exp + (obs-exp)2 /exp +(obs-exp)2 /exp X2=(4-4)2 /4+ (8-8)2 /8+(4-4)2 /4=0
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis.
EDWARD SYNDROME
The Edward's syndrome, which got its name after the famous doctor, Dr. John Edward (1960), is a genetic chromosomal disorder caused by an error in cell division resulting in an additional third chromosome 18. Edward's syndrome is manifested by a characteristic pattern of anatomical defects in a newborn child and has major implications for its health and survival.
Genetics:
Edward’s syndrome is characterized by the presence of an extra copy of genetic material on the 18th chromosome, either in whole (trisomy 18) or part (such as due to translocations). In majority of Edward's syndrome cases, all cells of the individual contain additional chromosome 18. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. With a translocation, the person has a partial trisomy for chromosome 18 and the abnormalities are often less than for the typical Edward’s. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edward’s syndrome.
Karyotype: 47,XY,+18
47,XX,+18
46,XX/47,XX,+18 (Mosaic condition)
Incidence Rate: Edward's syndrome is second most common after Down syndrome, occurs in approximately one among 3000 to 6000 births. The incidence rate increases as the mother's age increases. Signs and Symptoms: • Growth Deficiency, • Abnormal skull shape and facial features, • Clenched hands, • Rocker bottom feet, • Cardiac and renal abnormalities, • Horse shoe-shaped kidney, • Low set and deformed ears, • Prominent external genitalia, • Small placenta, • Mental retardation, • Hypotonia, • Microcephaly, • Micronagthia (abnormally small jaw), • Cleft lip, • Respiratory Failure -Apnea The survival rate of Edward’s Syndrome is very low, resulting from heart abnormalities, kidney malformations, and other internal organ disorders. About 95% die in utero. Of liveborn infants, only 50% live to 2 months, and only 5–10% will survive their first year of life. Major causes of death include apnea and heart abnormalities.
Aim: To verify Mendelian laws and study deviations from laws.
Observation: The observed ratio was 30:2.
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of duplicate genes, we hypothesize that the above case represents duplicate genes.
Theory: Gene duplication (or chromosomal duplication) is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination or duplication of an entire chromosome. The second copy of the gene is often free from mutations . So it will have no deleterious effects to its host organism. The two genes that exist after a gene duplication event are called paralogs and usually code for proteins with a different function and/or structure. Gene duplication plays a major role in evolution. Plants are the most prolific genome duplicators. For example, wheat is hexaploid (a kind of polyploid), meaning that it has six copies of its genome. There can be complete dominance at both gene pairs; however, when either gene is dominant, it hides the effects of the other gene.
e.g. Petal color in snapdragon plant.
P: AABB aabb
(Red colour) (white colour)
F1: AaBb
AaBb AaBb
One allele is sufficient to produce the pigment. Whenever a dominant gene is present, the trait is expressed. The phenotypic ratio observed in F2 generation in this case is 15:1.
Discussion: Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Since, the chi square vale is zero, we accept the abovementioned hypothesis. CRI DU CHAT SYNDROME
Cri du chat syndrome or Lejeune’s syndrome, was first described by Jérôme Lejeune in 1963. It is also called as 5p minus or 5p deletion syndrome. It is a group of symptoms that result from missing a piece of chromosome number 5. The syndrome’s name is based on the infant’s cry, which is high-pitched and sounds like a cat.
Genetics:
Cri du chat syndrome (CdCS) is due to a partial deletion of the short arm of chromosome number 5. Approximately 80% of cases results from a sporadic de novo deletion, while about 10-15% are due to unequal segregation of a parental balanced translocation, where the 5p monosomy is often accompanied by a trisomic portion of the genome. The phenotypes in these individuals may be more severe than in those with isolated monosomy of 5p because of this additional trisomic portion of the genome. Most cases involve terminal deletions with 30-60% loss of 5p material. Fewer than 10% of cases have other rare cytogenetic aberrations (eg, interstitial deletions, mosaicisms, rings and de novo translocations). The deleted chromosome 5 is paternal in origin in about 80% of the cases.
Karyotype: 46, XY, del [5p region]
46, XX, del [5p region]
Incidence Rate: 1 in 20,000 to 50,000 live births
Signs and symptoms:
➢ feeding problems because of difficulty swallowing and sucking ➢ low birth weight and poor growth ➢ severe cognitive, speech, and motor delays ➢ widely set eyes ➢ partial webbing or fusing of fingers or toes ➢ behavioral problems such as hyperactivity, aggression, tantrums ➢ unusual facial features which may change over time ➢ excessive dribbling ➢ constipation ➢ microcephaly(small head) ➢ growth retardation ➢ a round face with full cheeks ➢ low birth weight ➢ hypotonia ➢ epicanthal folds (folds of skin above eyes) ➢ down-slanting palpebral fissures ➢ flat nasal bridge ➢ down-turned mouth and high palate ➢ micrognathia(small jaw) ➢ low-set ears ➢ short fingers ➢ single palmar creases(simian crease) ➢ and cardiac defects(ventricular and atrial septal defects)
The Cri du chat affected people are fertile and can reproduce.
Less frequently encountered findings include:
➢ cleft lip and palate ➢ gut malrotation ➢ inguinal hernia ➢ dislocated hips ➢ cryptorchidism(undescended testis) ➢ rare renal malformations (eg horseshoe kidneys)
Late childhood and adolescence findings include:
➢ severe mental retardation ➢ microcephaly ➢ coarsening of facial features ➢ prominent supraorbital ridges ➢ deep-set eyes ➢ single line on the palm of hand (simian crease) ➢ Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time ➢ In males, testes are often small, but spermatogenesis is thought to be normal.
Aim: To verify Mendelian law and to study deviation from the law.
Observation: 9 red seeds; 6 brown seeds
Result:
Since the observed ratio is close to the phenotypic ratio observed in case of complementary genes, we hypothesize that the above case represents complementary genes. This case represents a deviation from Mendelian laws. Theory: Complementary Genes are those nonallelic genes, which independently show a similar effect but produce a new trait when present together in dominant form. Complementary genes were first studied by Bateson and Punnet (1906) in case of flower color of Sweet Pea (Lathyrus odoratus). Here, the flower color is purple if dominant alleles of two genes are present together (C-P-). The color is white if the double dominant condition is absent (ccP-, c-PP, ccpp ). If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure line, homozygous recessive plant with white flowers, the F1 plant will have colored flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but epistatic interactions of the C and P genes will give a modified 9:7 ratio. The following table describes the interactions for each genotype and how the ratio occurs. Genotype Flower Color Enzyme Activities 9 C_P_ Flowers colored; anthocyanin produced Functional enzymes from both genes 3 C_pp Flowers white; no anthocyanin produced p enzyme non-functional 3 ccP_ Flowers white; no anthocyanin produced c enzyme non-functional 1 ccpp Flowers white; no anthocyanin produced c and p enzymes non-functional
It is believed that the dominant gene C produces an enzyme which converts the raw material into chromatogen. The dominant gene P gives rise to an oxidase enzyme that changes chromatogen into purple anthocyanin pigment. This is confirmed by mixing the extract of the two types of flowerts when purple color is formed. Thus purple color formation is two step reaction and the two genes cooperate to form the ultimate product.
Raw material A Chromagen Anthocyanin
Gene C enz Gene P enz
Discussion:
Chi square value indicates the likelihood that the difference between expected and observed values occurs just by chance. Calculated chi square value is 0.07. Degrees of freedom =n-1= 2-1 =1 Chi square value from the table is 3.84
0.07<3.84
Since the calculated chi square value is less than that given in table at 5% level of significance, we accept the abovementioned hypothesis. DOWN’S SYNDROME
Down's syndrome, or trisomy 21 is a chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. It is the best known and most common chromosome-related disease syndrome formerly known as “mongolism”. It is named after Langdon Down, who first described its clinical signs in 1866.
Genetics:
About 95% of the cases are caused by non-disjunction, with most of the remainder being caused by chromosome translocation. The extra chromosome is contributed by the mother in 90-95% cases. About 75% of these maternal non-disjunctions occur during meiosis I, with the remainder occurring during meiosis II. There is a strong correlation between maternal age and the risk of producing a child with Down’s syndrome. Some patients with Down’s syndrome have a total of 46 chromosomes instead of 47, but in such cases a translocation has joined a part of long arm of chromosome 21 with the long arm of chromosome 14 (14q21q). Trisomy 21 is usually caused by nondisjunction in the gametes prior to conception, and all cells in the body are affected. However, when some of the cells in the body are normal and other cells have trisomy 21, it is called mosaic Down syndrome. This can occur in one of two ways: • a nondisjunction event during an early cell division in a normal embryo leads to a fraction of the cells with trisomy 21. • a Down’s syndrome embryo undergoes nondisjunction and some of the cells in the embryo revert to the normal chromosomal arrangement. There is considerable variability in the fraction of trisomy 21, both as a whole and among tissues. This is the cause of 1–2% of the observed Down syndromes.
Karyotype: 47,XY,+21
47,XX,+21
46,XX/47,XX,+21 (Mosaic condition)
Incidence Rate: Down’s syndrome is the most common autosomal aneuploidy seen among live births. It is seen in approximately 1/700 live birth, making it the most common aneuploidy condition compatible with survival to term. The risk for mothers less than 25 years of age to have the trisomy is about 1/1500; at 40 years of age, 1/1000; at 45, 1/400. Pregnant women over 45 are a special high-risk group.
Signs and Symptoms:
• Distinct facial features, such as a flat face, small ears, slanting eyes, and a small mouth. • A short neck and short arms and legs. • Weak muscles and loose joints. Muscle tone usually improves by late childhood. • Below-average intelligence. • Occipital is flat • Simian crease i.e. a deep flexion across the palms in 50% of the individuals. • Hypotonia is a highly consistent feature that is helpful in making a diagnosis. • Fertility amongst both males and females is reduced. Approx 75% of trisomy 21 conceptions are spontaneously aborted, the Down’s syndrome female’s risk of producing affected live-born offspring is considerably lower than 50%. • Heart, intestine, ear, or breathing problems. These health conditions often lead to other problems, such as airway (respiratory) infections or hearing loss. Most of these problems can be treated.
XYY SYNDROME
XYY syndrome or Jacob's syndrome is a rare chromosomal disorder that affects males. It is caused by an aneuploidy (trisomy) of the sex chromosomes, Y chromosome, thus a human male receives an extra Y chromosome in each cell. The first published report of a man with a 47,XYY Karyotype was by Avery A. Sandberg and colleagues at Roswell Park Memorial Institute in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence that was karyotyped because he had a daughter with Down syndrome.
Genetics:
Males normally have one X and one Y chromosome. However, individuals with Jacob's syndrome have one X and two Y chromosome. Males with Jacob's syndrome, also called XYY males. 47, XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in chromosome separation during metaphase I or metaphase II called no disjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells.
In some cases, the addition of an extra Y chromosome results from non disjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics
Karyotype: 47, XYY.
46, XY/47, XYY mosaics
Incidence Rate: About 1 in 1,000 boys are born with a 47,XYY Karyotype. The incidence of 47,XYY is not affected by advanced paternal or maternal age. Signs and Symptoms: Physical traits: Most often, the extra Y chromosome causes no unusual physical features or medical problems. • 47, XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height. • Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47, XYY. Testosterone levels (prenatally and postnatally) are normal in 47, XYY males. Most 47, XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason. Behavioral traits: • 47, XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. • As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings • Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47, XYY and are managed no differently than in 46,XY males. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
TRIPLE X SYNDROME
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The first published report of a woman with a 47,XXX karyotype was by Patricia A. Jacobs, et al. at Western General Hospital in Edinburgh, Scotland, in 1959.
Genetics: The condition is also known as triplo-X, trisomy X, XXX syndrome, and 47,XXX aneuploidy. Unlike other chromosonal conditions (such as fragile X), there is usually no distinguishable difference between women with triple X and the rest of the female population. Triple X syndrome is usually not inherited, but occurs as a random event during the formation of reproductive cells (ovum and sperm). An error in cell division called nondisjunction can result in reproductive cells with additional chromosomes. For example, an oocyte or sperm cell may gain an extra copy of the X chromosome as a result of the nondisjunction. If one of these cells contributes to the genetic makeup of a child, the child will have an extra X chromosome in each of her cells. In some cases, trisomy X occurs during cell division in early embryonic development. The additional X chromosome can come from either the maternal or paternal side. The condition is verified only by karyotype testing as it may not be distinguishable phenotypically. • Full trisomy: In this case, trisomy X occurs during gamete formation. • Mosaic trisomy: Some females with triple X syndrome have an extra X chromosome in only some of their cells. In this case, trisomy X occurs during embryogenesis.
Karyotype:
• Full trisomy: 47,XXX • Mosaic: 46,XX/47,XXX Incidence Rate: Triple X syndrome occurs in around 1 in 1,000 live female births. Signs and symptoms: • Due to inactivation and formation of a Barr body in all female cells, only one X chromosome is active at any time in a female cell and two Barr bodies are visible in somatic cell . Thus, triple X syndrome most often causes no unusual physical features or medical problems. • Females with the condition may have menstrual irregularities. • Although they rarely exhibit severe mental impairments, they have an increased risk of learning disabilities, delayed speech, and language skills. • a lanky/youthful appearance with increased facial beauty has been described, or in some instances varying degrees of androgeny, but these cases usually reflect traits present in near relatives. • Most women with triple X have normal sexual development and are able to conceive children. • A few may experience an early onset of menstruation. • Early menopause. • Triple X women are rarely diagnosed, apart from pre-natal testing methods, such as amniocentesis. Most medical professionals do not regard the condition a disability. However, such status can be sought by parents for early intervention treatment if mild delays are present.
TURNER’S SYNDROME
The syndrome is named after Henry Turner, an Oklahoma endocrinologist, who described it in 1938. The first published report of a female with a 45,X karyotype was in 1959 by Dr. Charles Ford and colleagues in Harwell, Oxfordshire and Guy's Hospital in London. It was found in a 14-year-old girl with signs of Turner syndrome.
Genetics: Turner’s syndrome encompasses several conditions, of which monosomy XO is the most common. Instead of the normal XX sex chromosomes for a female, (or XY for a normal male) only one X chromosome is present and fully functional; in rarer cases a second X chromosome is present but abnormal, while others with the condition have some cells with a second X and other cells without it (mosaicism). In Turner’ssyndrome, female sexual characteristics are present but generally underdeveloped. The risk factors for Turner’s syndrome are not well known. Nondisjunctions increase with maternal age, such as for Down syndrome, but that effect is not clear for Turner syndrome. There is currently no known cause for Turner syndrome, though there are several theories surrounding the subject. Karyotype: • Full Monosomy: 45,XO • Mosaic Monosomy: 46,XX/45,XO Incidence Rate: Approximately 98% of all fetuses with Turner syndrome result in miscarriage. Turner syndrome accounts for about 10% of the total number of spontaneous abortions in the United States. The incidence of Turner syndrome in live female births is believed to be 1 in 2500. Signs and Symptoms: -Short stature -Lymphedema (swelling) of the hands and feet -Broad chest , poor breast development and widely-spaced nipples -Low hairline -Low-set ears, hearing loss -Reproductive sterility, rudimentary ovaries, gonadal streak (underdeveloped gonadal structures) -Amenorrhea or the absence of a menstrual period -Increased weight, obesity -Shortened metacarpal IV (of hand) -Characteristic facial features,visual impairments -Webbing of the neck (webbed neck) -Congenital heart disease-Coarctation of the aorta -Horseshoe kidney -Normal skeletal development is inhibited due to a large variety of factors, mostly hormonal. -Due to inadequate production of estrogen, many of those with Turner syndrome develop osteoporosis. -Approximately one-third of all women with Turner syndrome have a thyroid disorder. -Moderately increased risk of developing diabetes. -Turner syndrome does not typically cause mental retardation or impair cognition. However, learning difficulties are common among women with Turner syndrome, particularly a specific difficulty in perceiving spatial relationships, such as Nonverbal Learning Disorder. -Women with Turner syndrome are almost universally infertile. Even when pregnancies do occur, there is a higher than average risk of miscarriage or birth defects, including Turner’s Syndrome or Down’s Syndrome. -Other symptoms may include a small lower jaw (micrognathia), cubitus valgus (turned-out elbows), soft upturned nails, palmar crease and drooping eyelids. Less common are pigmented moles, hearing loss, and a high-arch palate (narrow maxilla). Turner syndrome manifests itself differently in each female affected by the condition, and no two individuals will share the same symptoms.
