62 LINKAGE 
The three factors already studied, namely, white, 
miniature, and bar, furnish an excellent illustration. 
If we represent the percentages of crossing over as 
relative distances along the chromosome the three 
points will lie as shown in Fig. 25. 
If crossing over takes place between white and 
miniature and between miniature and bar, then it 
Fic. 25.—Diagram to illustrate double crossing over. The white and 
the black rods (a) twist and cross at two points. Where they cross they 
are represented as uniting (shown in c). That an interchange of pieces 
has taken place between W and Br is demonstrated by the factor M 
having gone over to the other chromosome. 
might be expected sometimes to take place in both 
regions at once, as shown in Fig. 25, b. The result here 
would be to produce two chromosomes like those 
shown in the lower figure. The combinations of 
factors which these two chromosomes resulting from 
double crossing over would contain, are white long 
bar and red miniature round. Since these two classes 
