CHROMOSOMES OF INDIAN RUNNER DUCK. 



349 



In the male there are seventy-six chromosomes. It would seem 

 then that seventy-four of these are autosomes and two are 

 Z-chromosomes. In maturation each sperm would get thirty- 

 seven autosomes, plus one Z-chromosome, or thirty-eight in all, 

 and all male gametes would therefore be alike. 



In the female there are seventy-seven chromosomes. Com- 

 paring these with the conditions in the male it would seem that 

 there are seventy-four autosomes, plus one Z, like those in the 

 male, plus two W-chromosomes which are not present in the 

 male. 



In maturation it might be supposed that the two W's become 

 linked together and that one of these might pair with the Z-chro- 

 mosome, the whole resulting in a tripartite body. If the thirty- 

 seven pairs of autosomes behave as in the male there would result 

 gametes in the female of two sorts (Figs. 55, 56) one having 

 thirty-seven autosomes plus the Z and the other having thirty- 

 seven autosomes plus the Ww pair. At the formation of the 

 zygotes then, those eggs having thirty-seven autosomes plus the 

 Z, fertilized by a sperm having thirty-seven autosomes plus a Z 

 would result in a zygote having seventy-four autosomes plus two 

 Z's or a male. Those eggs having thirty-seven autosomes and 

 two W's fertilized by a sperm as before having thirty-seven 

 autosomes plus a Z would result in a zygote having seventy-four 

 autosomes plus one Z and two W's or seventy-seven chromosomes 

 and would be a female. 



In sex-linkage the Z-chromosomes alone would be concerned. 

 If the double lined Z, represented in Fig. 55 as occurring in the 

 male, is thought of as carrying a dominant sex-linked character 

 and the single lined Z here represented as occurring in the female, 

 be thought of as carrying a recessive sex-linked character, then 

 in the FI generation, both males and females would show the 

 dominant sex-linked characters and would be like the male of the 

 parental generation. But in the FI generation all males would be 

 heterozygous for the sex-linked characters and would form 

 gametes of two sorts, half carrying the dominant sex-linked 

 character and half the recessive. 



In the F 2 generation all males would show the dominant sex- 

 linked character, but of the females of this generation, half 



