SEX INHERITANCE 87 



found that both the female and the male have 56 

 chromosomes, with no obviously unequal pair. 



Normall}^ in Abraxas the sex ratio is about 1 to 

 1. In one exceptional line this equality of sexes 

 was not the rule. In this strain Doncaster found 

 many females which gave only daughters, and not a 

 single son. Other females of this line gave many 

 daughters but also a few sons, while still others gave 

 practically a normal 1 to 1 ratio. 



When Doncaster examined this line cytologically, 

 he found that although the males were normal, with 

 56 chromosomes, the females were aberrant, having 

 only 55 chromosomes. 



In the maturation of the eggs of such a 55 chromo- 

 some female, the odd chromosome went to one pole, 

 so that one polar plate had 27 and the other 28 

 chromosomes. Doncaster found further that the odd 

 chromosome went more often to the polar body than 

 to the egg. The many eggs that ehminate the odd 

 chromosome become after fertihzation individuals 

 with 55 chromosomes, that is, females — while the 

 few that retain it become 56 chromosome individuals 

 — that is, males. The preponderance of the females 

 is thus accounted for. Such females having 55 chro- 

 mosomes would belong to the OZ type. 



In normal strains there is a W chromosome present, 

 but since this W chromosome may be absent without 

 effect upon the sex of the individual, as shown above, 

 it must be regarded as functionless in determining 

 sex, and in this sense it corresponds to the Y of 

 Drosophila. This evidence proves that there is 



