In Drosophila the normal eye colour is red. Morgan in one of his cultures found a white eyed individual. On breeding, this proved to be a pure line. When the white eyed flies were crossed with the wild (red) type, the results of reciprocal crosses were different i.e. white male x red female: white female x red male.
The result seemed to be dependent on the sex of the parent through which the trait was introduced into the cross rather than following the Mendelian pattern. Morgan assumed this to be due to the sex linked character of the gene for eye colour. The gene for normal eye colour (red) W is present on the X chromosome.
White eye colour is a recessive gene occupying the same locus (w). Hence for white eye colour to appear in the females the genes should be double homozygous (should be present in both the X chromosome). In the heterozygous condition in female (Ww) one X carries the W (red) gene, while the other X carries the w (white) gene. Phenotypically the heterozygous females are red eyed, but they are called carriers as they carry the recessive gene.
The males however cannot be carriers, for the gene for red or white being present on the only one X chromosome they have, their genotype and phenotype are one and the same. Even a single dose of the recessive gene (w) is enough to produce the white eye colour. This can be understood by the reciprocal crosses as follows.
ADVERTISEMENTS:
1. Red eyed male X White eyed female:
When a red eyed male (Wo) is crossed to a white eyed female (ww) in the F1 generation all the females would be red eyed and all the male individuals are white eyed. This is because the white eyed recessive gene is present on both the X chromosomes in the female parent.
The male offspring gets its X (w) from the mother and Y from the father. Since Y chromosome does not carry the gene for eye colour the individual would be white eyed. The female offspring (F1) having got one X (W) from the father and one X (w) from the mother would be heterozygous (Ww) genetically, but phenotypically show red eye colour.
When the F1 heterozygous female is crossed with the white eyed F1 male, in the F2 generation both red and white eyed (1:1) individuals are found in the female individuals, while in the male progency also red and white eyed individuals appear in equal proportion.
ADVERTISEMENTS:
Red eyed female x white eyed male:
When a pure red eyed female (WW) is crossed with a white eyed male (wo), the F1 offspring will have both females and males red eyed. The females however are carriers (heterozygous) as they have both Ww genes on their two X chromosome. When these two F1 individuals are crossed (heterozyous red eyed female X white eyed male ), among the female (F2) progeny (1) 50% will be red eyed (heterozyous) (2) 50% will be white eyed (homozygous double recessive). Among the male progeny (3) 50% will be red eyed and (4) 50% white eyed (see fig). The genotype of these flies will be as follows.
(1) Red eyed female (Ww)
(2) White eyed female (ww)
ADVERTISEMENTS:
(3) Red eyed male (Wo indicates Y chromosome which has no gene for eye colour)
(4) White eyed male (wo)