84 



MENDELISM 



I erect red, 2 erect purples, and i hooded purple. 

 And because the erect reds and the hooded purples 

 are respectively homozygous for E and B, they 

 must thenceforward breed true. The erect purples, 

 on the other hand, being always formed by the 

 union of a gamete Eb with a gamete eB, are always 

 heterozygous for both of these factors. They can, 

 consequently, never breed true, but must always give 

 erect reds, erect purples, and hooded purples in the 



EEbb eeBB Parents 



Eb 



Eb 



eB 



eB 



EeBb 



gametes 



EEbb ^— Eb 

 EeBb ^ — eB 

 EeBb ^—Eb 

 eeBB 



Fjj generation 



3 

 S- 



(D 



eB) "^ 



ratio I : 2 : I. The experimental facts are readily 

 explained on the assumption of repulsion between 

 the two factors B and E during the formation of the 

 gametes in a plant which is heterozygous for both.^ 



Other similar cases of factorial repulsion have 

 been demonstrated in the sweet -pea, and two of 

 these are also concerned with the two factors with 



1 In this account the repulsion between the two factors in question 

 has been assumed to be complete. Recent work, however, has shown 

 that this is probably not quite true. It is likely that the gametes EB 

 and eb are formed though only very rarely. For further discussion 

 the reader is referred to the Appendix at the end of this chapter. 



