DISTRIBUTION OF THE CHROMOSOMES 153 



some females would be produced that are XX YY, 

 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 XX Y female of Drosophila, would be 

 formed. Such a male would give rise to some sperm 



