62 LINKAGE 



The three factors already studied, namety, 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 



ir M Br 



(• • • •) 



a 





M 



Fig. 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 



