52 



INBREEDING AND OUTBREEDING 



If the description of the dual nature of the cells of 

 plants and animals and the result of gametogenesis is 

 recalled, the reason for the production of the ratio of 1 

 long spike : 2 intermediate spikes : 1 short spike in the F 2 

 generation is plain. Furthermore, it is clear why the 

 types like the grandparents breed true and the type like 

 the hybrid F 1 generation does not breed true. 



The long-spiked wheat has received the factor for long 

 spikes, the something in the germ cell that stands for the 

 production of long spikes, from both of its parents ; there- 



Pewale gawete 



Male gamete 



FIG. 17. Diagram showing the union of like gametes. 



fore it breeds true to long spikes. The gametes which it 

 produces all bear the factor for long ears. 



The diagram illustrating the fusion of the parental 

 gametes, shows why the long-spiked wheat produces 

 gametes, each of which bears the factor for long spikes. 

 If the letter S is substituted for the letter L in the dia- 

 gram, the same illustration holds for the short-spiked 

 wheat. But what happens when the long-spiked variety 

 is crossed with the short-spiked variety? A gamete bear- 

 ing L fuses with a gamete bearing S and a zygote LS is 

 formed. The interaction of the factors L and S produces 

 an F plant bearing intermediate ears. When this hybrid 



