INTERACTION OF FACTORS 



43 



white flowers in the ratio 9 : 7. And this, as we 

 have already seen, is what was actually found by 

 experiment. Further examination of the figure 

 shows that the coloured plants are not all of the 

 same constitution, but are of four kinds with respect 

 to their zygotic constitution, viz. A ABB, AABh, 

 AaBB, and AaBb. 

 Since AABB is 

 homozygous for 

 both A and B, all 

 the gametes which 

 it produces must 

 contain both of 

 these factors, and 

 such a plant must 

 therefore breed 

 true to the red 

 colour. A plant 

 of the constitution 

 AABb is homo- 

 zygous 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 

 consisting 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 therefore when bred from give 

 reds and whites in the ratio 9 : 7. The existence 



Fi<;. 7. 



Diagram to illustrate the nature of tlie Fo generation 

 from the two white sweet-peas which give a 

 coloured F"i. 



