SEX-LINKED HEREDITY 



281 



sophila and which also prevails in man has come to be called the 

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

 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 



Flies 



Chromosomes 



XY 



£XY 



c? 



9 

 Y cf 



X 9 



?xx 



9 



X Y 



X I 



X X 



J 1 



Parents 



Gametes 

 Fi 



Gametes 



XX XX XY XY Ft 



9 9 c? c? 



Fig. 70. — Reciprocal cross to that shown in Figure 69. Parents, red-eyed 

 male and white-eyed female; F I; white-eyed males and red-eyed females ("criss- 

 cross inheritance'" — Morgan); F 2 equal numbers of red-eyed and white-eyed in- 

 dividuals of both sexes. The distribution of the sex chromosomes is shown at the 

 right, as in Figure 69. (From Conklin, after Morgan.) 



with an X-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 



