V INTERACTION OF FACTORS 45 



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 interaction is necessary for the production of the 

 red colour, and as a cross between them brings these two 

 complementary factors together the Fj plants must all be 

 red. As this case is of considerable importance for the 

 proper understanding of much that is to follow, and as it 

 has been completely worked out, we shall consider it in 

 some detail. Denoting these two colour factors by A and 

 B respectively we may proceed to follow out the conse- 

 quences of this cross. Since all the F^ plants were red 

 the constitution of the parental whites must have been 

 A Abb and aaBB respectively, and their gametes conse- 

 quently Ah and aB. ,^^.^ ^^_. 

 ^ ■' White White 

 The constitution AAbb aaBB 



of the Fi plants 



must, therefore, be /* Ah aB aB ff^^^^t^ 



AaBb. Such a plant 

 being heterozygous 

 for two factors pro- 

 duces a series of ^ AB AB'« 

 gametes of the four £-zaB «-» „o 

 kinds AB, Ah, aB, | "* "* | 

 ah, and produces 



them in equal numbers (cf. p. 36). To obtain the 

 various t3^es of zygotes which are produced when such 



