THE MENDELIAN PRINCIPLES OF HEREDITY 97 



Gray Long Black Long Gray Vestigial 



2316 1146 737 



It will be noted that there are no black vestigial 

 flies. Their absence can be explained on the assump- 

 tion that no crossing over in the male, between the 

 factors in the second chromosome, has taken place. 



But if another generation (F s ) is raised, some black 

 vestigial flies appear. With these, it is possible to test 

 the hypothesis just stated. If, for instance, some of 

 the long, gray, F x females are mated to black vestigial 

 males, the following classes are produced : 



GL BL GV BV 



578 1413 1117 307 



The results are explicable on the view that crossing 

 over takes place in the germ-cell of the Fi female, 

 and that the chance that such will occur is as 1 to 3. 



But if the long-winged, gray, Fi males are crossed 

 to black vestigial females, only the following classes 

 are produced : 



BL GV 



992 721 



These results are in accord with the hypothesis that 

 no crossing over takes place between the second 

 chromosomes in the F x male. Why crossing over 

 should occur in the F x female, and not in the Fi 

 male, we do not know at present ; and until the 

 synaptic stages in the males and females have been 

 carefully studied, we must wait for an answer to the 

 question. 



