CHAPTER IV 
SEX INHERITANCE 
There are two types of sex inheritance known in 
those species in which separated sexes exist. In one 
type, which may be called the Drosophila type (XX- 
XY type, or, for short, the XY type), the female is 
homozygous for a sex factor, the male heterozygous; 
in the other, the Abraxas type (the WZ-ZZ type, or, 
for short, the WZ type) the female is heterozygous for 
a sex factor, the male homozygous. Since in both 
cases the heterozygous individuals must always mate 
with the homozygous ones there should result in each 
succeeding generation equal numbers of heterozygous 
and homozygous individuals, and so the bisexual con- 
dition is perpetuated as follows: 
Ss 
| /\ 
Ce / 
The genetic evidence so far gained has placed in the 
Drosophila type the following animal forms: Dro- 
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