THE STRENGTH OF LINKAGE 



371 



and the way they are distributed by segregation in the 

 two crosses under discussion. 



The Strength of Linkage. In the last paragraph 

 male hybrids of the vinegar fly were back-crossed to 

 jemale recessives because the linkage of genes in the 

 chromosomes of male vinegar flies is complete, i.e., there 

 is never any separation of the genes in the same chromo- 

 some, and consequently the characters that originally 



Sperm mother cell of 



Parent I 



Gray 



Long 



GL GL 



(Note, only 

 oneoftne 

 four chro- 

 mosome pairs 

 is shown) 



Egg mother cell of 



Parent II 



black 



vestigial 



gl gl 



A sperm of 



Parent I 



GL 



® 



The Gray Long Hybrid 

 (male) 

 GL gl 



An egg of 



Parent 11' 



gl 



® 



Gray Long 

 GL 



Sperms of 

 the hybrid 



(black vestigial) 



fi! 



Fig. 107. — (a) Showing the segregation of linked genes in gray 

 long hybrid of type GL gl. 



went into the cross came out together again without fail 

 in the second hybrid generation. This peculiarity of 

 linkage in male vinegar flies, viz., that linked characters 

 never separate, is not true for the female hybrids or for 

 males of all species of animals. By mating a female 

 (Fig. 108) hybrid Gray Long vinegar fly of type (1) (i.e., 

 the offspring of Gray Long and black vestigial) with a 



