2O4 T. H. MORGAN. 



These results are in harmony with the hypothesis of no crossing 

 over in the male between genes in the third group of chromo- 

 somes. The amount of crossing over between pink and kidney 

 in the female is about 15 per cent., which is greater than the 

 most frequent value of about 5 per cent, in Sturtevant's case of 

 pink ebony. 



DISCUSSION. 



In their paper of 1911, Bateson and Punnett described certain 

 phenomena that they called coupling and repulsion. The numer- 

 ical and class results offer many points of similarity to the cases 

 here described. In a later paper they gave reasons for abandon- 

 ing the earlier view of repulsion as distinct from coupling. In this 

 paper (December, 1911) they gave two cases in which linkage oc- 

 curs, but in which crossing over must be assumed to take place 

 in both sexes. Bateson and Punnet postulate in order to account 

 for their results that segregation takes place at some early stage 

 in the germ-tract. Whatever form of interpretation may apply 

 to the cases described by Bateson and Punnett, the tests that 

 I have made of FI males and females show, that in Drosophila at 

 least, the results are due to failure of crossing over between factors 

 in one sex. It may be that in certain animals and plants crossing 

 over is the same in both sexes, while in other cases it may be 

 that crossing over is different in the two sexes. Whether this 

 is true or not can only be determined by making tests like those 

 here employed. Until such tests have been made for other forms 

 in which linkage has been found we cannot know how widely the 

 explanation here followed may be extended. 



