GENETICS PURPLE EYE COLOR DROSOPHILA 



289 



In carrying out the triple back cross, the principle of balancing 

 the inviability by complementary crosses was applied. To com- 

 pletely balance a three locus experiment required four types of 

 crosses, so that every class may appear in each of the four 

 crossover categories, namely, (0) non-crossovers, (1) crossovers 

 in the first region, that between black and purple, (2) crossovers 

 in the second region, that between purple and vestigial, and 

 (1,2) double crossovers, the simultaneous occurrences of crossing 

 over in both regions. Thus, the cross of black purple vestigial 

 by wild and the back cross of the Fi wild-type daughters by the 

 triple recessive male gave one of the four types of crosses 

 (table 13). The other types of experiment carried out were 

 black by purple vestigial (table 14), black vestigial by purple 

 (table 15), and black purple by vestigial (table 16). In order 



TABLE 13 

 Pi mating, black purple vestigial d^ X wild female; B. C. mating Fi wild-type 9 X 



black purple vestigial cf 



TABLE 14 

 Pi, black X purple vestigial; B. C. test of Fi 9 9 singly 



