EYE COLOR IN DROSOPHILA MELANOGASTER 363 



Fo ratio was again 12:3: 1, as in similar crosses with other re- 

 cessive specific dilutors. But the creams (cream b) which oc- 

 curred in this F2 were not as pale as any of the preceding creams. 



From the circumstances of the appearance of cream b — that 

 it was observed in the Fi of an outcross and as a single individual 

 — we should expect it to be a dominant, but as a matter of fact 

 it proved to be a recessive. It seems probable, in explanation, 

 that more creams were actually present in this Fi but were over- 

 looked, since attention was distracted by the simultaneous appear- 

 ance in the same culture of still another mutation (lethal 4), and 

 more especially since the effect of cream b is rather slight. Only 

 occasionally was one of the F2 creams as marked as the grand- 

 father, and the mutation might not have been recognized at all 

 were it not that an extreme fluctuant had attracted attention. 

 Since cream b is recessive, we must suppose that the gene was 

 present in both parent stocks. It could have been present in 

 the bar stock and been undetected because of the lack of eosin, 

 without which it has no visible effect. And the character might 

 readily have been present in the eosin non-disjunction stock and 

 have been passed over as an age variation, since as we ordinarily 

 see flies from a stock culture they are of all ages and of corre- 

 sponding densities of pigmentation. 



A pure breeding stock of cream b was made up for use in back 

 crossing. By this time we were in possession of a good second 

 chromosome dominant 'star' and likewise of a perfect third 

 chromosome dominant 'dichaete,' which mutants have now be- 

 come the most important in their respective chromosomes. By 

 aid of these two dominants it is very easy to determine in a single 

 experiment whether a given mutant is in the second or third 

 chromosome. Thus, in the case of cream b, a stock of eosin 

 star dichaete was made up and used in making a Pi cross to the 

 cream. Then Fi eosin males which showed both star and di- 

 chaete and which were heterozygous for the recessive cream were 

 backcrossed to cream b females of stock. There is no crossing 

 over in the male of Drosophila, so that if cream b were in the 

 second chromosome, none of the B.C. (back cross) stars should 

 be cream, while half of the dichaete should be cream and half 



