262 



PHYSICAL BASIS OF HEREDITY 



one pure for dicbete and the one pure for lethal. The sur- 

 viving classes continue to produce the same kind of 

 offspring since they are, like the parents, heterozygous 

 for the two lethal factors. But the factors are not near 

 enough together to prevent crossing over, which occurs 

 in about 5 per cent, of cases between the lethal and peach 

 genes. The next diagram, Fig. 114, shows how when 

 crossing over takes place in the female, there result four 



Crossover eggs cr.*iv,T« 

 (5fo of total) ^P^^^ 



If 



95+95+5 Dichete 

 5 Dichete Peach 



} =97 % 



2Vj 



DHp 



SliB 



IXI 



Fig. 114. — Diagram illustrating crossing over of factors shown in Fig. 113. 



classes (see squares), of which two die (as before), and 

 of the two that survive one is dichete peach. Taking both 

 non-crossover and crossover results together, the expec- 

 tation is 95 + 95 + 5 dichete to 5 dichete peach or 971/0 

 to 21/2. This stock then breeds true for dichete without 

 showing the gene it carries for peach eye-color except in 

 a small percentage of cases, and if the peach-eyed fly 

 should be unable to establish itself in nature, like some 

 of the (Enothera mutants, the stock would not be changed 

 by it, but continue to throw off a few "mutants'' with 

 peach-colored eyes. 



