Essay on the Sex- linked inheritance in Drosophila

Genes are said to be sex-linked when they are present on X chromosome and’ phenotypic ally expressed in case of organisms with XY mechanism of sex determination, this a part of X chromosome for which there is no homologous region on the Y.

In XY males, get earned on the nonhomologous part of X-chromosome show expression even if they are pre in recessive condition. Such genes are called sex-linked genes.

Sex- linked inheritance in Drosophila:

Red eye character of Drosophila is dominant over white eye. All offsprings of eyed males and red eyed females in the Fj generation are red eyed. The F, hybrids when allowed

To produce F₂ offspring’s through inbreeding, the red and white eyed flies are found in a ratio of 3:1 following Mendelian principle.

The data when classified on the basis of sex and eye colour produce a peculiar situation.

i) A red eyed female, when bred with white eyed male, produced all red eyed F, hybrids, irrespective of sex.

ii) The reciprocal cross yields F, hybrids with red eyed females and white eyed males in equal proportion.

iii) F₂ progeny, produced by inbreeding of F, hybrids under the former cross (cross i), shows all females with red eye. From among the male offsprings, 50 per cent were with red eye and other 50 per cent possessed white eye.

iv) F₂ progeny, produced by inbreeding of F, hybrids under the later cross [reciprocal cross (ii)], shows both males and females with red eye and white eyes in equal proportion. The crosses and the results can be summarised in.

The peculiar pattern of inheritance can be explained through the analysis of genotypes with sex chromosomes. Drosophila sex determination is through XX and XY mechanism. The gene for eye colour is present on the X chromosome. The homologous Y chromosome does not bear the gene due to lack of homologous region corresponding to respective region of X. Therefore, the female flies will have double doses of the gene for eye colour. If red eye colour is denoted by the capital alphabet W and that of white eye by the small alphabet w, the genotypes for female flies are WW, Ww, and ww. Corresponding genotypes of male flies will be with only one dose of the gene i.e. W or w.

The genotype of true breeding red eyed female can be denoted as X^WX^W and white eyed male as X” Y. The red eyed female will produce only one type of egg (X^w), while the white eyed male will produce two types of sperms (X^w and Y). By random union the genotypes, the Fj will be either X^WX^W female or X^w Y male.

As the dominant allele is present in female and the only allele for the white eye colour is present in male, all F, progenies, irrespective of male and female, are red eyed. F, progenies undergoing inbreeding produce F₂ progeny with all females being red and half of the males being either red or white as depicted in.

In the reciprocal cross, the white eyed female with the genotype X^WX^W produces only one type of gamete X^w carrying the gene for white eye colour. The red eyed male X^w Y produces two types of gametes one with X^w chromosome having gene for red eye clour and the other with Y chromosome not having any gene for eye colour. When, they unite at random, produce females (X^WX^W), which are with red eye due to presence of dominant allele and males (X^WY), which are with one dose of the recessive gene producing white eye. F₂ progeny produced by inbreeding ofF, flies show males and females with both red and white eye in equal proportion as depicted in.