286 



THE CONTINUITY OF THE RACE 



W to represent the gene for round and w to represent the gene for 

 wrinkled; G to represent the gene for yellow and g the one for green. 

 Consequently, the genetic formula for the two Pi individuals may be 

 written GG WW for the yellow-round parent and gg ww for the green- 

 wrinkled parent. 



Diagram of Mendel's Dihybrid Cross 



Phenotypes of two parent races Yellow-round X green-wrinkled 



Their genotypes GG WW gg ww 



Their gametes, either male or female G W g w 



Consequent genotype of the F\ Gg Ww 



Observed phenotype of the F\ Yellow-round 



Kinds of gametes produced by F\ if independent 



assortment occurs GW, Gw, gW, gw 



All the possible combinations between the four types of male and of 

 female gametes produced by the F\ are shown in the following "checker- 

 board" diagram. Each of the 16 squares gives the genotype that would 

 result from one of the possible combinations of gametes and the resulting 

 phenotypic appearance of the plant. 



We find that Mendel's second assumption — the occurrence of inde- 

 pendent assortment — appears to fit and so "explain" his experimental 

 results. If the dihybrid produces four kinds of male gametes in equal 

 numbers and produces the same four kinds of female gametes in equal 

 numbers, and if all possible combinations of male and female gametes 

 take place according to the laws of chance, nine kinds of genotypes will 



