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Indiana University Studies 



test is an absolute demonstration of the presence of a factor 

 for bristle number in the third chromosome, since extra bristle 

 number is only partially dominant to the normal. I think 

 it is indicative, however, and when taken in conjunction with 

 the fact that the pink-eyed flies in the cross of the selec- 

 tion line to black, pink, bent, have very few extra bristles, 

 it becomes even probable that there is a factor for extra 

 bristles in the third chromosome. Crosses of the selection 

 line were made to sooty by itself and also to spineless. The 

 Fo sooty and spineless flies were similar to the F. pink-eyed 

 flies in bristle number. 



The selection line was also crossed to black purple curve 

 plexus speck, the genes of which are in the second chromo- 

 some. The F/s were mated inter se and also the F^ females 

 were mated back to black purple curve plexus speck males. 

 Crossing-over occurred in this case, but no significant differ- 

 ence in the bristle number of the various types of cross- 

 overs was observed. The selection line was also crossed to 

 another mutant provisionally called ''star", a lethal which 

 affects the arrangement of the ommatidia of the eye. The 

 gene for this character, according to Sturtevant, is near the 

 zero end of the second chromosome. The F/s were inbred. 

 The distribution of the bristles was practically the same for 

 the two types which appeared in Fo. 



A further test for the fourth chromosome was made with 

 eyeless. The Fo e^-eless and normal flies, however, were simi- 

 lar when compared with respect to bristle number. 



In all of the crosses of the mutant races to the selection 

 line, with one exception, the normal or wild type flies in Fo 

 had a slightly higher bristle number than those of the mutant 

 race. In the case of bent, as shown in the cross of the selec- 

 tion line with black, pink, bent, the mean bristle number of 

 the bent flies is slightly greater than the mean of any of 

 the other types. Also the percentage of normal flies is less. 

 It is possible, of course, that the gene for bent influences 

 bristle number. 



While my main conclusion that multiple factors are opera- 

 tive in the production of extra bristles agrees with the con- 

 clusion of MacDowell, yet some of the other conclusions do not 

 agree. In the development of his interpretation, MacDowell 

 assumes the presence of a primary inhibitor which keeps the 



