GENETICS PURPLE EYE COLOR DROSOPHILA 



281 



TABLE 6 



The B. C. offspring given by the Fi wild-type S07is, from the outcross of a purple 

 male to a vestigial female, token hack crossed to purple vestigial females 



The second point attacked was the amount of crossing over in 

 the female between the loci purple and vestigial. Fi daughters 

 from the same two complemetery crosses that had furnished the 

 material for the male tests just given were back crossed singly 

 to purple vestigial males. 



BALANCED INVIABILITY— COMPLEMENTARY CROSSES 



The reason why both 'coupling' and 'repulsion' experiments 

 were made is that by combining the two sets of data one can 

 calculate a linkage value more nearly free from the errors due 

 to disproportionate inviability of any class (Bridges, '15; Muller, 

 '16). Within each back cross the inviability effects due to a 

 given mutant form are largely neutralized. Since the inviable 

 form occurs both as a crossover and as a non-crossover, both of 

 these classes are lowered, but lowered proportionately, so that 

 the linkage ratio remains practically undisturbed. This internal 

 balancing holds less well for combinations of characters, for any 

 given combination occurs in an experiment either as a crossover 

 or as a non-crossover, but not as both, and should any combi- 

 nation have an inviability disproportionate to that of the com- 

 ponent mutant forms, then the crossover value would be 

 disturbed. The remedy for this condition is to balance the 

 experiments in which a relatively invariable class occurs as 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 28, NO. 2 



