366 



CALVIN B. BRIDGES 



gene for pinkish is not in the second chromosome. While this 

 was a mistaken notion — the true relation being that the gene is 

 so far away from black that in the female there is entirely free 

 crossing over — yet it led to the device of the efficient 'double 

 mating' method of ridding a given stock of an undesired recessive. 

 If pinkish were in the third chromosome, then the presence 

 of the black in the pinkish stock could be of no advantage, and 

 might be a very serious handicap, since it would prevent the use 

 of all our third chromosome stocks containing ebony or sooty. 

 The first step in the elimination of black was to mate together 

 some of the not-black pinkish flies of table 22, One-third of 

 the not-black offspring of such pairs should be of the desired 



TABLE 22 



The offspring given by the Fi eosin-eyed daughters from the outcross of black pi7ikish 

 females to eosin males, lohen back-crossed to black pinkish males 



kind, that is, entirely free from black. Our task was then to 

 pick out from the mixture of pure grays and grays heterozygous 

 for black some pure gray males for outcrossing to eosin females. 

 In this special case we were aided by the fact that black happens 

 to be slightly dominant, that is, the heterozygous blacks are 

 somewhat darker than the pure grays. While this difference 

 is not marked enough to be used regularly in classification, yet 

 it enables us to pick out by inspection a greater proportion of 

 pure grays than we would get by random selection. Four such 

 males were selected as being probably free from black and were 

 mated to eosin females. Into the same bottle with each pair 

 of these flies was put a virgin (red-eyed) black female. The 

 offspring from these two mothers are easily distinguished, since 



