Cytogenetic Proof of Crossing Over 205 



the cytological basis of crossing over that had not been found 

 previously. 



Stern's work in Drosophila was very similar (Fig. 62). Here 

 two X chromosomes were heteromorphic. In one strain, which 



Fi female 



car - 

 B- 



+ - 



testcross male 

 car- 



X 



+ - 



N 



females 



car - car - 



B- +H 



males 



car 



+ - car - 



+ - 



B- 



K 



Offspring 



phenotypes 



carnation S 

 bar 



+ 



K 



red 

 nonbar 



parental 

 combinations 



car 



+ 



+ H 

 B 



car 



+ 



car 



+ 



car 



+ 



K 



H-- 



B- 



h 



carnation ^ 

 nonbar 



red 

 bar 



* recombinations 



Fig. 62. Stern's method of demonstrating genetic crossing over cyto- 

 logically by means of a translocation in the X chromosome of Drosophila 

 melanogaster. For explanation, see text. 



carried the genes car (carnation-colored eye) and B (bar-shaped 

 eye), a piece of the X chromosome had broken off from the B 

 end. This piece had become attached to one of the members 

 of chromosome IV so that its genes were not lost. In the other 

 strain, a piece of the Y chromosome had broken off and become 

 attached to the car end of the X chromosome; it carried the 



