80 SEX INHERITANCE 
not present in all species. In all these cases the 
female contains two X chromosomes, the Y chromo- 
some being confined to the male line. 
This type of sex determination represents all eggs 
as alike—each containing one X (after the polar 
bodies have been extruded), but the sperm is of 
two kinds, one containing the X and the other Y, or 
merely no X. The scheme is as follows: 
| /\ 
X Y 
X XY 
It wil) be seen that all the spermatozoa carrying X 
produce females, while all those carrying Y or no X 
produce males. 
The Y chromosome, when present, descends from 
father to son. It might seem, therefore, that if the 
Y carried a sex factor for maleness the scheme would 
work out as well as if a sex factor were carried by the 
X chromosome. But in several cases there is no Y 
in the male, and in certain cases to be described later, 
due to non-disjunction, there are females that have a 
formula XXY and yet their sex is not affected in any 
way on account of the presence of the supernumerary 
Y. It follows that sex is not determined by the 
presence or absence of the Y chromosome but by the 
