GENETICS PURPLE EYE COLOR DROSOPHILA 



287 



of the next generation. Likewise, if one of the purples had 

 come from a crossover black purple egg the black X purple 

 cross would produce some blacks that would give the required 

 black purples upon inbreeding. If both the black and the 

 purple chosen happened to have come from crossover eggs, 

 then the double would be produced in F3 directly. In case none 

 of the parents proved to be from crossover gametes then at 

 least the F3 wild-type flies are equivalent to the Fi and would 

 save a generation in the repetition. The other two types of 

 crosses would give a favorable result only if both parents 

 happened to be from crossover eggs, in which case the double 

 would appear among their progeny. It so happened that one 

 of the black X black crosses gave a few black purples in F3 

 directly, and from these a stock was made for use in back 

 crossing. At the same time a Pi mating of a black male to a 

 purple female was started to furnish the required Fi heterozy- 

 gotes. A single test of the Fi male showed, as expected, no 

 crossing over whatever in the male (table 11). 



Two back-cross tests of the female gave a total of 773 flies 

 of which 38, or 4.9 per cent were crossovers (table 12). Of the 

 two expected values, that of 30 is excluded entirely, and that of 



TABLE 11 

 Pi mating, piirple d^ X black 9 ', B. C, Fi c^ X black purple 9 



TABLE 12 

 Fi mating, purple cf X black 9 ', B. C, Fi 9 X black purple cf 



