260 



PHYSICAL BASIS OF HEREDITY 



stock in this condition alone should give 10 per cent, of 

 crossing over, i.e., it should still produce a small percentage 

 of normal flies. It so happened, however, that there was 

 present in the stock a third gene that lowers the amount 

 of crossing over in the female to such an extent that, for 

 the two '^distances'' here involved, practically none takes 

 place. When it does, a normal fly appears, but tliis is 

 so seldom that such an occurrence, if it happened in a 

 domesticated form of which the wild type was unknown, 



Crossover 

 Eggs 



Sperm 



1, N-- 



B B-- 



--ll 



--N 



IBead , INorm .0.5^ 



NH 



KB 



liB 



liM 



Fig. 112. — Diagram showing the results of crossing over in a stock containing both beaded 

 and lethal, as shown in Fig. 111. 



would be set down as a mutation like that shown by the 

 evening primrose. 



The third factor that entered into the result is not 

 unique, for Sturtevant has shown that crossover factors 

 are not uncommon in Drosophila. The analysis that Mul- 

 ler has given for beaded, while theoretical, is backed up 

 by the same kind of genetic evidence that is accepted in 

 all Mendelian work. It makes an assumption but one that 

 can be demonstrated by any one who mil make the neces- 

 sary tests. It is also possible to produce at will other bal- 

 anced lethal stocks that will '^ mutate" in the sense that 



