

THE FACTOR HYPOTHESIS 153 



stands for the absence of the purple pigment gene. It 

 will be seen that each of the white sweet peas the for- 

 mulse of which are given above lack one of the two 

 essential factors for purple color. When the two are 

 crossed, however, all the progeny are purple with the 

 formula CcPp. 



These hybrid sweet peas upon gametic segregation 

 theoretically produce four kinds of gametes, CP, Cp, 

 cP, and cp which may combine as any other dihybrid 

 in sixteen different ways. In this case, however, these 

 combinations group themselves into only two pheno- 

 types, purple and white, as indicated in the accom- 

 panying diagram (Fig. 33) in which C and c are repre- 

 sented by large and small circles respectively, while 

 P and p are correspondingly indicated by large and 

 small crosses. The gametic symbols are superimposed 

 to form the zygotes. 



The theoretical expectation here shown was closely 

 approximated in the actual results. 



It may be noted in passing that the seven kinds of 

 white sweet peas resulting from the above cross, while 

 phenotypically alike, that is, in the zygotic symbols 

 of Figure 33, lacking either the large circle (color) or 

 the large cross (pigment), belong to three distinct groups 

 of genotypes as follows : 



