Sex-Linkage 



A-l B-l 



w w 

 X Y 



99 



x'"y"99 



A-2 B-2 



p, XX yxxvu p, xxyxXvU 



F, F^ i 



x">99 x">9? 



w w 

 X X 



A-3 



9 

 x->99 



w w 

 X X Y 



^ 



w w 

 X X 



B-3 



+ 



9w w 

 X X Y 



c/ 



W W 



X Y 



c/c/ 

 *x-99 



FIGURE 12-4. Three attempts (A-l and B-l, A-2 and B-2, A-3 and B-3) 

 to represent matings A and B in Figure 12-3 genotypically. Shaded genotypes 

 must be incorrect. 



reciprocal cross (Figure I2-5B) of barred 

 99 X nonbarred cf cf all sons are barred and 

 all daughters nonbarred. Here also the re- 

 sults of reciprocal matings differ, so that we 

 are dealing again with sex-linkage. Note 

 from the second cross that the exceptional 

 individuals are the Fi that show the recessive 

 trait, as was the case in Drosophila. But in 

 the present case the sex is opposite, since the 



Fi which are nonbarred are females, whereas 

 the exceptional Fi Drosophila were white-eyed 

 males. In order to explain these results we 

 can assume, as in Drosophila, that sex is 

 determined by XX vs. XY, that the X chro- 

 mosome does, and the Y chromosome does 

 not, contain a gene for barred or nonbarred 

 feathers, but that, contrary to the situation in 

 Drosophila, males are XX and females XY. 



