134 THEORY OF EVOLUTION 



Black Gray Black Gray 



vestigial long long vestigial 



83% 17% 



The combinations that entered are more com- 

 mon in the F 2 generations than the cross-over 

 classes, showing that there is linkage of the fac- 

 tors that entered together. 



Another curious fact is brought out if in- 

 stead of back-crossing the FI female we back- 

 cross the FI male to a black vestigial female. 

 Their offspring are now of only two kinds, 

 black vestigial and gray long. This means 

 that in the male there is no crossing-over or 

 interchange of pieces. This relation holds not 

 only for the Group II but for all the other 

 groups as well. 



Why interchange takes place in the female 

 of Drosophila and not in the male we do not 

 know at present. We might surmise that when 

 in the male the members of a pair come to- 

 gether they do not twist around each other, 

 hence no crossing-over results. 



Crossing-over took place between white and 

 yellow only once in a hundred times. Other 

 characters show different values, but the same 



