V INTERACTION OF FACTORS 47 



constitution AABb is homozygous for the factor A, but 

 heterozygous for B. All of its gametes will contain A , but 

 only one-half of them will contain B, i.e. it produces equal 

 numbers of gametes AB and Ab. Two such series of 

 gametes coming together must give a generation consist- 

 ing of X AABB, 2x AABb, and x AAbb, that is, reds 

 and whites in the ratio 3:1. Lastly the red zygotes of 

 the constitution AaBb have the same constitution as the 

 original red made from the two whites, and must there- 

 fore when bred from give reds and whites in the ratio 

 9:7. The existence of all these three sorts of reds was 

 demonstrated by experiment, and the proportions in 

 which they were met with tallied with the theoretical 

 explanation. 



The theory was further tested by an examination into 

 the properties of the various Fg whites which come from a 

 coloured plant that has itself been produced by the mating 

 of two whites. As Fig. 7 shows, these are, in respect of 

 their constitution, of five different kinds, viz. AAbb, Aabb, 

 aaBB, aaBb, and aabb. Since none of them produce any- 

 thing but whites on self-fertilisation it was found neces- 

 sary to test their properties in another way, and the 

 method adopted was that of crossing them together. It 

 is obvious that when this is done we should expect differ- 

 ent results in different cases. Thus the cross between two 

 whites of the constitution AAbb and aaBB should give 

 nothing but coloured plants ; for these two whites are of 



