THEORY OF EVOLUTION 139 



The Evidence from Non-Disjunction 



In the course of the work on Drosophila ex- 

 ceptions appeared in one strain where certain 

 individuals did not conform to the scheme of 

 sex hnked inheritance. For a moment the 

 hypothesis seemed to fail, but a careful exami- 

 nation led to the suspicion that in this strain 

 something had happened to the sex chromo- 

 somes. It was seen that if in some way the X 

 chromosomes failed to disjoin in certain eggs, 

 the exceptions could be explained. The analy- 

 sis led to the suggestion that if the Y chromo- 

 some had got into the female line the results 

 would be accounted for, since its presence there 

 would be expected to cause this peculiar non- 

 disjunction of the X chromosomes. 



That this was the explanation was shown 

 when the material was examined. The females 

 that gave these results were found by Bridges 

 to have two X's and a Y chromosome. 



The normal chromosome group of the fe- 

 male is shown in figure 52 and the chromosome 

 group of one of the exceptional females is 

 shown in figure 69. In a female of this kind 



