52 LINKAGE 



about five to one that the combination that went in 

 holds together. 



It is also instructive to repeat the cross in such a 

 way that the two mutant factors, black and vestigial, 

 enter from different sides, i.e. , one parent contributes 

 black and the other vestigial. As shown in the next 

 diagram (Fig. 22), each parent carries in its chromo- 

 some one mutant factor and the normal allelomorph 

 of the other. 



If the Fi males are backcrossed to black vestigial 

 females only two classes result, viz., black long and 

 gray vestigial, Fig. 22 (to the left) . These are the 

 combinations that entered ; hence no crossing over has 

 taken place in the FI males. We see that here the 

 linkage is not due to some affinity between the factors 

 black and vestigial, per se, for in this cross they always 

 enter different gametes as surely as they stayed 

 together before. The reason for this difference in 

 result is that in this cross they came from different 

 parents and must have been in opposite chromosomes, 

 whereas in the previous cross they were in the same 

 chromosome. 



If we test the FI females by mating to black ves- 

 tigial males, four classes result, viz., 



Non-crossovers Crossovers 



Black, long Gray, vestigial Black, vestigial Gray, long 



41.5 per cent. 41.5 per cent. 8.5 per cent. 8.5 per cent. 



83 per cent. 17 per cent. 



Crossing over has taken place in the FI females, 

 and the numerical results show that this happens in 



