272 



CALVIN B. BRIDGES 



the results of the first were fully known. A purple vestigial 

 male outcrossed to a wild female produced wild-type sons and 

 daughters (page B39; + 9 15, + c^ 10). Four of the Fi 

 females were back crossed each by two or three purple vestigial 

 males from stock. In this case Fi females happened to be 

 chosen because, as is usually the case, they hatched somewhat 

 earlier than their brothers in the same culture. 



These back-cross cultures (table 3), in common with the 

 previous F2 cultures (table 2), showed a fair amount of crossing 

 over between purple and vestigial. A calculation showed that 

 the percentage of crossing over was 9.1. 



TABLE 3 



The B. C. offspring given by the Fi daughters, from the outcross of a purple vestigial 

 viale to a ivild female, when back crossed to purple vestigial males 



This was recognized as being of a different degree from the 

 apparent percentage of 1.8 calculated from the first back cross 

 (table 1). It was now realized for the first time that the two 

 back crosses had differed in the sex of the Fi flies tested by the 

 back crosses — that the first back cross was a test of the amount 

 of crossing over in the male and the second was of crossing over 

 in females. Up to this time there had been no suspicion that the 

 result of a back cross could be in any way dependent on the sex 

 of the Fi parent used in the experiment. From this evidence it 

 was concluded that there was crossing over in the male, but that 

 it was of different degree from that in the female. In Septem- 

 ber, 1912, Morgan showed that in the case of black vestigial no 

 crossing over whatever had occurred in the male, while in the 

 female there was even more crossing over than had been found 



