GENETICS PURPLE EYE COLOR DROSOPHILA 275 



this double crossing over occurred within a region only ten units 

 long — a space shorter than that in which double crossing over 

 of the ordinary type has ever been detected even in certain re- 

 gions of the autosomes in which double crossing over is relatively 

 most frequent. 



MUTATIONS 



Two new mutations were found and two old ones reoccurred 

 in these back-cross experiments on the linkage of purple and 

 vestigial. 



'Kidney' eye shape, a third chromosome recessive, was found 

 in B. C. culture B10.2, June 26, 1912 (table 1). This mutant, 

 the first affecting the shape or texture of the eye, was con- 

 siderably used in the early days (Morgan, '14, and Bridges, 

 '15), but has now been superseded by mutants less variable and 

 easier to classify. 



In culture B39.2 it was noticed, July 26, 1912, that several 

 of the wild-type flies had more dorsocentral bristles on the 

 thorax than the regular number, four. Later it was found that 

 such extra-bristled flies were occurring in small proportions in all 

 four sister cultures, from which it would appear that the muta- 

 tion was a recessive, introduced through the purple vestigial 

 stock used twice in the experiment. The extra bristles oc- 

 curred among all classes in the experiment indifferently, which 

 would seem to .indicate that the gene were not second chromo- 

 some, since if it were the extras should have been relatively 

 more frequent among the purple vestigials. The number of 

 extra bristles varied from one to four, the highest total bristle 

 number observed being eight. Extra bristles were also observed 

 to be frequent in two or three other stocks. A stock throwing 

 extra thoracic bristles derived from B39.2 was maintained by 

 careless mass selection for some time and was finally given to 

 Mr. E. C. MacDowell to be used as the basis of rigorous selection 

 experiments (MacDowell, '15). As the result of a survey of all 

 stocks known or suspected to contain extra bristles, MacDowell 

 chose a certain wild stock as the most favorable starting-point 

 for his selection. 



