Inheritance of X-Y- Linked genes

INHERITANCE OF X-Y-LINKED GENES

The genes that occur in the homologous section of X and Y chromosomes have inheritance like the autosomal genes. The X-Y linked genes are partially or incompletely sex-linked because sometimes,the crossing over may occur in the homologous sections of X and Y chromosomes. In humans several diseases are XY-linked.Certain XY-linked genes of man are of total colour blindness,two skin diseases (Xeroderma pigmentosum and Epidermolysis bullosa).Retinitis pigmentosa,etc.

SEX-LINKED LETHAL

Certain X-linked genes are lethal,that is they cause the death of an individual from egg up to sexually mature adult stage.X linked lethal have been reported both in Drosophila and human beings.

1. Sex-linked lethal in Drosophila.In Drosophila,rare females regularly produce female and male progeny in the ratio of 2:1 which indicates that half of the male progeny is missing.This peculiarity can be explained due to the presence of a recessive sex-linked lethal gene in the heterozygous condition in the female parent .
2. Sex-linked lethal in humans.In fact, the gene for haemophilia is a recessive sex-linked lethal since it may cause death.Slight scratches in normal persons may result in fatal bleeding for the haemophiliac.

Duchenne muscular dystrophy is another fatal disease of humans.In it affected individual though apparently though apparently normal in early childhood exhibits progressive wasting away of the muscles,resulting in confinement to a wheelchair by about the age of 12 years and death in teen years .

SEX INFLUENCED GENES

Sex influenced genes are those whose dominance is influenced by the sex of the bearer. Thus,male and female individuals may be similar for a particular trait but give different phenotypic expressions of the same trait. Thus,here the same allele has different expressions in male and female individuals but both alternative forms of the trait are known as in each sex,i.e.,both horned and hornless sheep are known as female and male.

3.Further in cattle also sex-influenced genes occur as spotting in cattle.The genes for mahogany and white spotting is dominant in males and then the genes for red and white for red and white spotting is dominant in females.

SEX LIMITED GENES

Sex-limited genes are autosomal genes whose phenotypic expressions is determined by the presence or absence of one of the sex hormones.Their phenotypic effect is limited to one sex or other.In other words, the penetrance of a sex-limited gene in one sex remain zero.The sex-limited genes are mainly responsible for secondary sex characters in cattle,humans, and fowl.

NON-DISJUNCTION

The failure of homologs or sister chromatids to separate properly to opposite pole is called nondisjunction.The sex chromosomes pass through the phenomenon of nondisjunction and present various interesting situations. The nondisjunction may be following types:

1. Primary Non-disjunction

The phenomenon of primary nondisjunction was discovered by C.B.Bridges(1916)during his classical one has classical crosses as follows:

The gene for wild-type red eyes (+)is carried by the X chromosomes;a recessive allele (v)produces vermilion eyes in homozygous females (X^VX^V)and in all males (X^VY).Normally vermillion eyed females mated to red eyed males produces only red-eyed daughters and vermilion eyed sons.

The metafemales (AA XXX)are weak and seldom live beyond the pupal stage;the AAOY individual dies in the egg stage.Note that the AA XXY female indicate that the presence of Y chromosomes does not determine maleness itself though males without it (AAXO)are sterile.From the results, three facts became established:1.In Drosophila sex determination occurs according to genetic balance theory of Bridges.2.In Drosophila,Y chromosomes contain genes for the spermatogenesis.3.It provides strong support to chromosomes theory of inheritance.

2.Secondary Nondisjunction

In next cross, bridges mated the exceptional vermilion eyed females(AAX^VX^VY)that arose as a result of primary nondisjunction to normal red-eyed males(AAX⁺Y).The progeny of this cross included 96 percent female with red eyes and only 4 percent females had vermilion eyes indicating that there was 4 percent secondary nondisjunction.

Thus,primary nondisjunction may occur in either XX females or XY males.In the former, it leads to the production of XX and O eggs.Occurrence in the first meiotic division of males produces XY and O sperms.When such nondisjunction takes place during the second meiotic division then XX,and XY and O sperms result.Secondary non-disjunction, on the other hand, occurs in WWY females,where it gives rise to XX,XY,X and Yeggs.As the term nondisjunction implies,these aberrant gametes are produced only as a result of the failure of the sex chromosomes to disjoin after synapsis;they are not physically attached.

Similarly, results were obtained by bridges (1916)from the cross of red eyed male and white eyed female Drosophila.

References

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Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.