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^ plants 

 were red the constitution of the parental whites 

 must have been AAbb and aaBB respectively, and 



their gametes 

 White White consequently Ab 



AAbb aaBB , d t-u 



and aB. i he 



^ ^ ' - I constitution of 



Ah Ab aB^ «^ofTarents the Fj plants 



must, therefore, 

 be AaBb. Such 

 a plant being 

 heterozygous for 

 two factors pro- 

 duces a series of 

 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^ 

 generation look for the appearance of coloured and 



