Heredity and Environment 



these characters occur, thus giving rise to a larger number of 

 types of individuals than when a single pair of characters is 

 concerned. When there is only one pair of contrasting characters 

 there are three genotypes and usually but two phenotypes in the 

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



F, 



YRCGW) 



X YR YW GR GW 



9^ 



YR 



YW 

 GR 

 GW 



RN 



YW 

 YR 



YR 



F 2 



FIG. 31. DIHYBRID DIAGRAM SHOWING RESULTS OF CROSSING PEAS HAV- 

 ING YELLOW-ROUND (YR) SEEDS WITH OTHERS HAVING 'GREEN-WRINKLED 

 (GW) ONES. The hybrids of the first filial generation (F t ) 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 of these 

 $ and $ gametes are shown in the small squares. Of these 16 combina- 

 tions 7 contain the same letters (factors) so that there are only 9 differ- 

 ent genotypes, and since recessive characters do not appear when mated 

 with dominant ones these 9 genotypes produce only 4 phenotypes in the 

 following relative numbers: 9 YR:$ YW:$ GR: I GW. There is I pure 

 dominant (upper left corner), I pure recessive (lower right corner) ; 

 4 homozygotes in the diagonal line between these corners, and 12 heterozy- 

 gotes. 



