200 DISTRIBUTION OF THE CHROMOSOMES 
some females would be produced that are XXYY, 
owing to the union of an XY egg with an XY sperm 
or an XX egg with a YY sperm. One such female 
was found—she had two X and two Y chromosomes. 
Here then is a case that seemed at first to be in 
direct contradiction to the scheme of sex linked 
inheritance based on the chromosome hypothesis, 
which proved, however, on further examination to 
give a brilliant confirmation of that theory; for not 
only can the hereditary results be accounted for, but 
the theory on which they were based was directly 
confirmed by a microscopical study of the chromo- 
somes themselves. 
Cases indicating non-disjunction have also been 
obtained in Abraxas, by Doncaster. _As stated in 
the chapter on Sex Inheritance, he has found a strain 
in which the males have 56 chromosomes—the 
normal number, but the females have only 55 instead 
of 56 chromosomes. It seems reasonable, then, to 
suppose that such females arose by the passing of the 
two sex chromosomes, ZZ, to one pole (spermatocyte) 
leaving none at the other pole of the cell. The sperm 
resulting from the no-Z cell fertilizing a Z egg would 
give a ZO individual which would be a female with 
55 chromosomes. All the daughters of the ZO 
female would be ZO and her sons ZZ individuals: 
and the race would continue in this fashion. On 
the other hand, if the ZZ sperm produced by non- 
disjunction fertilized a W egg, a male WZZ, corre- 
sponding to the X XY female of Drosophila, would be 
formed. Such a male would give rise to some sperm 
