In his original studies of Figure 420 Morgan first suggested

In his original studies of Figure 4.20, Morgan first suggested that the original white- eyed male had two copies of the white-eye allele. In this problem, let’s assume that he meant the fly was XwYw instead of XwY. Is the data in Figure 4.20 consistent with this original hypothesis? What additional cross could you do to rule out the out the possibility that the Y chromosome carries a copy of the eye color gene?

Cross A Cross C X X All progeny red-eyed X WY X W Y I X X X WY Cross B Cross D wt w XT X 3 red XTX X WY XT X Crisscross inheritance XWY 1 white

Solution

Morgan was the first to discover sex-linkage in drosophila.The sudden appearance of one white eyed male is due to a mutant gene present in the sex chromosome which becomes recessive in the next generation to give rise all red-eyed males and females. The data in fig. 4.20 is not consistent with the original hypothesis of Mendel as in the inheritance of eye colour the Y chromosomes does not play any role.

Therefore if we keep on doing an intercross between male and female where Y chromosomes have no mutant allele of eye colour it proves that Y chromosomes genes are not essential for the sex-linked inheritance for eye-colour.