88 



Plant Genetics 



getting one with only one of the determiners. There 

 would be just 25 per cent of each of the type represented 

 in the diagram, the resulting ratio in the F^ being 



9:3:3:1- 



F, No Coupling 



4 Possible Gametes 



Fig. 21. — Diagram showing how nonnal dihybrid segregation, in 

 absence of coupling, produces four different gametes. 



In the case we are considering, however, the mecha- 

 nism is different. At the reduction division of the Fi 

 generation two chromosomes with the dominant de- 

 terminers are linked together by a bond. In conse- 

 quence of this there are only two types of gamete 



F, Coupling 



Only 2 Gametes 

 Possible 



These 2 Gametes 

 Impossible, Due to 

 Presence of Bond 



Fig. 22. — Diagram showing how coupling would limit possible 

 kinds of gametes. 



possible, as indicated in the diagram (fig. 22). In the 

 terminology of cytology the two chromosomes with 

 the determiners are held so firmly together that they 

 pass to the same pole of the spindle. This is the phe- 

 nomenon of coupled inheritance and a theoretical 

 mechanism that was devised to explain it. 



