42 MENDELISM CHAP. 



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 

 consequences of this cross. Since all the F a plants 

 were red the constitution of the parental whites 

 must have been A Abb and aaBB respectively, and 



their gametes 



White Wgg consequently Ab 



/\ y\ and aB. The 



/ \ / \ constitution of 



' .. D r> eametes 



Ab Ab aB # nf narpnts fV,^ F 



_ , 



must, therefore, 



Red r-i 



be AaBb. Such 

 AaBb 



a plant being 



heterozygous for 



\ 



Ab Ab u7 two factors pro- 



p's a -? a -f duces a series of 



ab ab "3 



S % gametes of the 



four kinds AB, 



Ab, aB, ab, and produces them in equal numbers 

 (cf. p. 33). To obtain the various types of zygotes 

 which are produced when such a series of pollen 

 grains meets a similar series of ovules we may make 

 use of the same " chessboard " system which 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 or B alone, or 

 neither. In other words, on this view of the nature 

 of the two white sweet -peas we should in the F 2 

 generation look for the appearance of coloured and 



