CHAPTER lY
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:
 

The genetic evidence so far gained has placed in the
Drosophila type the following animal  forms: Dro-
 

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