162 GENETICS 



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 

 phenotypes, purple and white, as indicated in the 

 accompanying diagram (Fig. 49) in which C and c 

 are represented by large and small circles respec- 

 tively, 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 49, lacking either the large circle (color) or 

 the large cross (pigment), belong to three distinct 

 genotypes as follows : — 



1 



2 

 3 



Without the pigment factor (large cross) 



Without the color factor (large circle) 



Without either pigment (large cross) or color (large circle) 



Number of 

 Zygote in 

 Figure 49 



6- 8-14 

 11 -12 15 



16 



Among the purple peas are the following four geno- 

 types : — 



