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- 



n 



n 



t^- 



D 



Fig. 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 



