SEX INHERITANCE 81 
number of the X chromosomes that are present. In 
the cases that follow, where sex determination of the 
Drosophila type was discovered by a study of sex 
linked inheritance, as well as in the above cases, where 
the mechanism was discovered through cytological 
observations, proof that the male is heterozygous for a 
Mendelian factor for sex is derived from the fact that 
he gives rise to two kinds of spermatozoa—male pro- 
ducing and female producing—in equal numbers. 
We know this in the cases worked out cytologically 
because here the spermatozoa carrying X must all 
produce females, while the other half must produce 
males; and we know it, in the cases worked out gen- 
etically, because here only half the spermatozoa from 
a male with a dominant sex linked character carry 
the dominant factor, and these all produce females, 
while the rest produce males. The female must con- 
tain the same Mendelian sex factor as is present in 
the female-producing spermatozoa of the male; but 
the female must be homozygous for this factor, since 
any egg, if fertilized by a male-producing sperma- 
tozoon, contributes this factor to the resulting male. 
Although the only way in which the results of sex 
linked inheritance of the Drosophila type differ 
from non-sex linked cases is the one above stated, 
namely, that a dominant male transmits his dominant 
sex linked factor only to his daughters, nevertheless 
it may be well at this point to recall specifically what 
ratios are produced in consequence, in the various 
types of crosses. 
Examples of sex linked inheritance in Drosophila 
