246 



THE SECOND-CHROMOSOME GROUP 



crosses with other recessive specific dilutors. But the creams (cream 

 b) which occurred in this F 2 were not as pale as any of the preceding 

 creams. 



From the circumstances of the appearance of cream b, viz, that it was 

 observed in the FI of an out-cross and that 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 overlooked, since attention 

 was distracted by the simultaneous appearance 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 F 2 creams 

 so marked as the grandfather, 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 all corresponding densities of pig- 

 mentation. 



CHROMOSOME CARRYING CREAM b. 



A pure-breeding stock was made up for use in back-crossing. By 

 this time we were in possession of a good second-chromosome domi- 

 nant "star" and likewise of a perfect third-chromosome dominant 



TABLE 93. B. C. offspring from the PI mating of an eosin star dichcete male to 

 a cream b female and the back-crossing of the FI eosin star dichcete male 

 to cream b females. 



"dichaete," which mutants have now become 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 dichsete was made up and used in making a PI cross to the 

 cream. Then FI eosin males which showed both star and dichsete and 

 which were heterozygous for the recessive cream were back-crossed 



