HERITAGE OF THE INDIVIDUAL 



275 



gametes bearing the same and for gametes bearing different 

 genes to meet in fertilization. 



The zygotes are 1 SS : 2 Ss: 1 ss. But, since S is dominant, 

 the resulting organisms 

 will be in the proportion 

 of 3 tall to 1 dwarf, which 

 is the familiar 3: 1 Men- 

 delian ratio of dominants 

 to recessives in the F 2 

 generation. The import- 

 ant point, however, is 

 that these tall organisms, 

 although they all appear 

 alike or, as we now say, 

 belong to the same 

 PHENOTYPE, are different 

 with respect to their germ- 

 inal constitution; because 

 one third bear germ cells 

 all of which contain the 

 gene S, . and two thirds 

 bear germ cells half of 

 which contain S and 

 the other half s. Conse- 

 quently the phenotype is 

 composed of two GENO- 

 TYPES which are distin- 

 guishable only by what 

 they produce. (Fig. 139.) 



It is thus apparent why 

 the pure tall plants (ex- 

 tracted dominants) al- 

 ways breed true, and why 



FIG. 139. Diagram of a Mendelian mono- 

 hybrid. Results of crossing large size (S) and 

 small (s) Pea plants. The circles represent the 

 zygotes and the characters of the soma (pheno- 

 type); the letters within the circles, the ger- 

 minal constitution (genotype). The letters out- 

 side the recombination square represent the gam- 

 etes. Note that each of the parents (P) represents 

 a different phenotype and genotype; all the FI 

 (one shown) belong to the same phenotype and 

 genotype ; while the F 2 represent two phenotypes 

 and three genotypes. The relative number of 

 individuals composing the Fa phenotypes is 3 : 1 . 



