346 



CAL^^X B. BRIDGES 



A similar experiment in which cream was crossed to eosin 

 ebony (ebony being a third chromosome mutant, Sturtevant, 

 '14'i gave a t>7)ical 9:3:3:1 ratio (table 5). which agrees with 

 the fact that the cream gene is not in the third chromosome. 



In order to find the locus of cream within the second chromo- 

 some, it would have been necessary to run linkage experiments in 

 which all the flies were eosin; thus, one of the experiments might 

 have been cream II by eosin black and a backcross of the Fi 

 female to black cream males, and another a similar backcross in 

 which cur^'ed was used in place of black. The amount of crossing 

 over between black and curved was known to be about 25 per 

 cent. The two values black cream and curved cream which 

 would be found by two such experiments (both values might, of 



TABLE 5 

 Fi from the cross of cream II cf by eosin ebony 9 



course, be found from a single more comphcated experiment) 

 would enable the locus of cream to be calculated with considerable 

 accuracy. TMiile much is to be learned of the mechanism of 

 crossing over from a study of the relative distributions of loci 

 within various regions of the chromosome, 3'et in the case of cream 

 II it was thought that the compensation would not be worth the 

 effort. .\ny further use of cream II in other linkage experiments 

 would involve the 'eosinization' of aU the stocks used. In the 

 case of certain of the later creams, an approximate location of the 

 gene within the chromosome has been made, but such location 

 was made less laborious by the discovery of certain dominant 

 mutations which were not available at the time the work on cream 

 II was finished. 



