4 SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 



When the vestigial fly was crossed to normal the mutant char- 

 acter disappeared in the hybrid. If such a hybrid is outbred to 

 normal all the offspring are again normal, but half of them carry 

 the vestigial gene. If these are outcrossed again still only normal 



Fig. 2. Cross between long and vestigial wings, giving long in Fi. The 

 long-winged hybrid Fi female is then represented as out-bred to a wild-type 

 male, giving long-winged offspring again — half pure-long and half hybrid- 

 long. The last are represented as again out-bred to wild type, giving long- 

 winged offspring again and of the same two genetic kinds as above. 



flies appear. Fig. 2. If such outbreeding is continued the vestig- 

 ial gene will become widely distributed without ever showing it- 

 self at the surface, so to speak. If, however, at anv time two 



