MUTATION 



259 



pair of genes that almost no crossing over takes place 

 between the levels occupied hy the two pairs. These rela- 

 tions are illustrated in the next diagram, Fig. 111. Here 

 again the two pairs of vertical lines to the left represent 

 the two third-chromosome pairs in the female and to the 

 right in the male. The location of the two pairs of genes 

 involved, N-l^ and B-N, are indicated. These combina- 

 tions give the four classes in the squares of which two 

 classes die, viz., NNBB (pure for beaded) and Ijl^NN 



Eggs 



Sperm 



N-- 



B-- 



H B 





All Beaded 



NB 



NB 



liS 



liR 



Fig. 111. — Diagram to show how the appearance of a lethal near beaded causes 

 the stock to produce only beaded except for the small number of crossovers, as shown 

 by the next diagram. 



(pure for lethal three). The result is that only beaded 

 flies come through, and since all these are heterozygous 

 both for B and l^, the process is self -perpetuating. 



If the preceding account represented all of the facts in 

 the case, the stock of beaded should have bred perfectly 

 true, but it has been shown in Drosophila that crossing 

 over between the members of the pairs of genes takes 

 place in the female. Hence we should expect a complica- 

 tion due to crossing over here unless the level of the two 

 pairs of genes was so nearly the same as to preclude this 

 possibility. In fact, in addition to the beaded flies the 



