SEX DETERJMINATION AND SEX-LINKED HEREDITY 



413 



Drosophila type of sex-linkage. There is, however, quite a diflferent 

 type that is called the poultry type, which, while strikingly like the type 

 already described, differs from it in one important respect. 



The poultry type of sex linkage. — In the Drosophila type, the 

 female is the homozygous sex (producing only one kind of gamete, each 



Fig. 82. — Sex-linked inheritance of barred and unbarred (black) plumage in 

 poultry. P, parents, barred male, unbarred female; Fi, barred males and females; 

 F2, males all barred, females in equal numbers barred and unbarred. {After 

 Morgan.) 



with anX-chromosome), and the male is heterozygous (producing two 

 kinds of gametes, one with an X- and one with a Y-chromosome) . 

 Now certainly in moths and butterflies, and probably in birds, the male 

 is homozygous and the female heterozygous. It is the custom to 

 designate the sex-chromosome condition as WW for the male and WZ 

 for the female, though why we should not use XX and XY it is difficult 

 to say. With this reversal of sex-chromosome composition of the two 

 sexes we might expect that sex-linked heredity would work out just 



