92 



Heredity and Environment 



types of individuals than when a single pair of characters is 

 concerned. When there is only one pair of contrasting characters 

 there are usually but two types of offspring apparent in the F 2 

 generation, viz., dominants and recessives in the ratio of 3 : I 

 (Fig. 30) ; where there are two pairs of contrasting characters 

 in the parents there are four types of offspring in the F 2 genera- 



YRCGWJ 



F* 



Fig. 31. Dihybrid Diagram Showing Results of Crossing Peas Hav- 

 ing Yellow-round (YR) Seeds with Others Having Green-wrinkled 

 (GIV) Ones. The hybrids of the first filial generation (F 2 ) are all yellow 

 and round since these characters are dominant while green and wrinkled 

 are recessive, YR(GW). Four types of germ cells are formed by such a 

 hybrid, viz., YR, YW, GR, GW, and the 16 possible combinations (geno- 

 types) of these $ and ? gametes are shown in the small squares. Since re- 

 cessive characters do not appear when mated with dominant ones these 16 

 genotypes produce 4 phenotypes in the following relative numbers : 9 YR : 

 3 YW : 3 GR:i GW. There is 1 pure dominant (upper left corner), 1 

 pure recessive (lower right corner), 4 homozygotes in the diagonal line 

 between these corners, and 12 heterozygotes. 



