HEREDITY 327 



and Green (y) and Yellow Is dominant over Green. Characters 

 for seed smoothness were Smooth (S) and Wrinkled (s) and 

 Smooth is dominant over Wrinkled. The inheritance of either pair 

 may be considered alone, in which case they are exactly like the first 

 example of a monohybrid cross. But when considered together the 

 number of genotypes and phenotypes of the Fi> generation is 

 greater than in a monohybrid cross. If it is assumed that in a pair 

 of chromosomes the gene Y is on one and y on the other, and that 

 a second pair of chromosomes contain the genes S and s, then in the 

 Fi generation all the phenotypes are Yellow and Smooth, for these 

 are the dominant characters. When the germ cells of the F] genera- 

 tion mature, each may contain one of the following combinations, 

 since each contains one member of every pair of chromosomes: 

 YS, Ys, yS, ys. In the fertilization of the eggs which produce the F2 

 generation, the possible combinations are shown in the accompany- 

 ing diagram (Fig. 205). The letters in the columns across the top 

 represent the possible types borne by the tgg, those in the col- 

 umns along the side represent the genes borne by the sperms. Each 

 block within the diagram represents a possible association of genes 

 in an offspring, that is, its genetic constitution. It is conventional to 

 represent dominant characters by capital letters and recessives by 

 small. From the diagram it is clear that there are phenotypically 12 

 Yellow (Y) and 4 Green (y), which is the 3:1 Mendelian ratio for 

 a single pair of characters. Likewise there are^u Smooth (S) and 4 

 Wrinkled (s). Counting all phenotypes, there are 9 Yellow (Y) 

 Smooth (S), 3 Yellow (Y) Wrinkled (s), 3 Green (y) Smooth 

 (S), and I Green (y) Wrinkled (s). Thus in a dihybrid cross the 

 F2 generation extends the 3:1 ratio of phenotypes of a monohybrid 

 to 9:3:3:1. It should be clear that these figures represent ratios, 

 that is, the distribution of characters among the offspring. Reliable 

 ratios are obtained only when there is a large number of offspring 

 in a generation. With small numbers reliable ratios do not appear. 

 For example, if one tosses a coin a great number of times, the num- 



