44 MENDELISM chap. 



phenomena which result from such interaction between 

 separate and distinct factors. 



One of the most interesting and instructive cases in 

 which the interaction between separate factors has 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 noth- 

 ing 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, genera- 



White X White , . ' . , , , , . 



I tion consists of reds and whites, 



Red F, the former being rather more 



I 1 1 numerous than the latter in the 



^9? ^f^^*^""^^ proportion of 9:7. The rais- 



ing of a further generation 

 from the seeds of these Fj 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 



