GENETICS PURPLE EYE COLOR DROSOPHILA 273 



in the case of purple vestigial. Subsequent tests, including 

 hundreds of thousands of individuals, have shown that ordi- 

 narily there is no crossing over in the male for any chromosome 

 and that the few cases that have occurred were probably not 

 brought about by the same mechanism as that by which cross- 

 ing over is ordinarily effected. 



NO CROSSING OVER IN THE MALE 



A clear conception of the fact of no crossing over in the male 

 was prevented in the original vermilion purple vdltigial back- 

 cross test by the apparent occurrence of crossovers in one of the 

 five lines. No tests were made of the apparent crossovers 

 because there was at that time no evidence, aside from the in- 

 consistency within the experiment, to suggest that they were 

 highly unusual. Against the supposition that some clerical 

 error might have been made is the strong internal evidence pre- 

 sented by the aberrant cultures. Thus, the cultures could not 

 have been F2's that were mislabeled, since the proportion of 

 purple vestigials in this line is the same as that in the other back- 

 cross cultures and is much larger than that in any of the F2 

 cultures. Also, the parents were carefully examined when they 

 were transferred to the third culture bottle and were seen to be 

 a vermilion male and vermilion purple vestigial females only, 

 which is the back-cross type of mating. The examination of the 

 parents also excluded the supposition that the line may have 

 been a back-cross test of the female rather than of the male. 

 Perhaps some unknown peculiarity of the stocks used may have 

 been responsible for the apparent crossing over. Thus, it has 

 been suggested that some other eye color resembling purple, 

 such as 'maroon,' had been present, probably only in hetero- 

 zygous form, in the vermilion purple vestigial stock. Such an 

 explanation would account for the crossover class classified as 

 vermilion purple, but entirely fails to account for the comple- 

 mentary class of exceptions — the few but carefully attested ves- 

 tigials that were not-purple. In fact, none of the suggestions 

 that have been made have offered a satisfactory escape from the 

 alternative of some kind of crossing over in the male. 



