46 



MENDELISM 



CHAP. 



a series of pollen grains meets a similar series of ovules 

 we may make use of the same "chessboard" system 

 whjch we have already adopted in the case of the fowls' 

 combs. An examination of this figure (Fig. 7) shows 

 that 9 out of the 16 squares contain both A and B, while 7 

 contain either A ox B alone, or neither. In other words, 

 on this view of the nature of the two white sweet peas we 

 should in the Fj generation look for the appearance of 



coloured and 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. AABB, 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 mi^st 

 therefore breed true to the red colour. A plant of the 



Fig. 7- 



Diagram to illustrate the nature of the Fj generation 

 from the two white sweet peas which give a 

 coloured Fi. 



