V INTERACTION OF FACTORS 41 



been demonstrated is a case in the sweet-pea. All 

 white sweet -peas breed true to whiteness. And 

 generally speaking the result of crossing different 

 whites is to produce nothing but whites whether in 

 F^ or in succeeding generations. But there are 

 certain strains of white sweet -peas which when 

 crossed together produce only coloured flowers. The 

 colour may be different in different cases, though for 

 our present purpose we may take a case in which the 

 colour is red. When such reds are allowed to self- 

 fertilise themselves in the normal way and the seeds 

 sown, the resulting F^ 

 generation consists of reds *^^^^^ ^ White 



andwhiteSjthe former being p d- -- -F 



rather more numerous than 1 



the latter in the proportion „r"! ~~~i 



. rj., ^ .^ . Red White- ---F2 



ot 9:7. 1 he raismg 01 a |g\ ^y^ 



further generation from the 



seeds of these F^ plants shows that the whites always 

 breed true to whiteness, but that different reds may 

 behave differently. Some breed true, others give reds 

 and whites in the ratio 3:1, while others, again, give 

 reds and whites in the ratio 9 : 7. As in the case 

 of the fowls' combs, this case may be interpreted in 

 terms of the presence and absence of two factors. 

 Red in the sweet-pea results from the interaction of 

 two factors, and unless these are both present the 

 red colour cannot appear. Each of the white 

 parents carried one of the two factors whose inter- 

 action is necessary for the production of the red 

 colour, and as a cross between them brings these 

 two complementary factors together the F^ plants 

 must all be red. As this case is of considerable 



