60 LINKAGE 
period, the homologous chromosomes unite in pairs. 
There has been much controversy as to how this 
union takes place, but in some cases at least, the 
uniting chromosomes twist around each other as 
they come together. This is illustrated to the left 
in Fig. 24. As a consequence, parts of one chromo- 
A B Cc D 
Fic. 24.—Diagram to represent crossing over. At the level where the 
black and the white rod cross in A, they fuse and unite as shown in D. 
The details of the crossing over are shown in B and C. 
some will come to lie now on one, now on the other 
side of the mate. If when the twisted chromosomes 
separate, the parts on the same side go to the same 
pole the end result will be that shown to the right 
in Fig. 24. Each chromosome has interchanged a 
part with its mate. This process has been called 
crossing over. It is, of course, also possible that the 
twisted chromosomes do not break and reunite where 
