GENETICS PURPLE EYE COLOR DROSOPHILA 



295 



'ALTERNATED' BACK CROSSES 



In cases where only one type of back cross is to be made the 

 poorest type is that in which all the mutant genes are in the same 

 homologue as was the case in the black purple curved X wild 

 experiment just cited. The flies having the most mutant char- 

 acters are relatively the least viable, and this type of cross 

 furnishes the highest proportion of such individuals. The best 

 type is that known as 'alternated/ where the successive genes 



( b + c ) 



alternate between the two chromosomes 



so that the 



+ pr + 



maximum of evenness of distribution of characters is attained. 

 It required double crossing over to put all the mutant characters 

 in the same individual, and accordingly the 'alternated' experi- 



TABLE 19 

 The partially balanced back-crosses involving streak, purple, and curvcl 



ment gives a minimum number of the combination that is most 

 inviable. This principle becomes still more important in more 



complex experiments, as, for example, -, 7-. 



^ +d+pr + px + 



The next mutant whose locus was mapped with reference to 

 purple as a base was 'streak,' a dominant character which 

 shows as a dark streak from the scutellum forward along the 

 dorsal region of the thorax. The triple back-cross streak purple 

 curved, which was made in two of the four possible ways, and is 

 therefore partially balanced (table 19), showed that streak is 

 far to the left of purple, that is, beyond black, and in the oppo- 

 site direction from vestigial and curved. The streak purple 



