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, *AABb, 



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 



Ab, 

 Ab 



ab 



\ 



aB 

 aB 



Ab 



aB 

 ab 



rabi 

 'ab 



FIG. 7. 



Diagram to illustrate the nature of the F 2 generation 

 but from the two white sweet peas which give a 



, c coloured Fj. 



heterozygous tor 



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 



