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, 7.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 F,; males are backcrossed to black vestigial 
females only two classes result, viz., black long and 
gray vestigial Fig. 22 (third line). These are the 
combinations that entered; hence no crossing over has 
taken place in the F, 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 F, 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 F, females, 
and the numerical results show that this happens in 
