INHERITANCE 



291 



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

 resulting organisms will be in the ratio of 3 tall to 1 dwarf, which 

 is the familiar 3 : 1 Mendelian ratio of dominants to recessives in 

 the F 2 generation. The important point, however, is that these 

 tall plants, although they all appear alike and therefore belong to 

 the same phenotype, are actually different with respect to their 

 germinal constitution ; because one-third bear gametes all of which 

 contain the gene S, and two-thirds bear gametes half of which con- 

 tain S and the other half s. Consequently the tall phenotype is 



• • 



o o 



x o 



o 



%8 



•To 



F, 



• • 



• o 



o o 



Fig. 182. - - Inheritance of color in a cross between pure black (pigmented) 

 and white (albino) Guinea-pigs. The small circles represent the genes involved. 



composed of two genotypes which are distinguishable only by 

 what they produce. (Figs. 181, 182.) 



It is thus apparent why the pure tall plants always breed true, 

 and why the dwarfs (necessarily pure) do the same — all the 

 gametes of one bear S and those of the other, s. The pure plants 

 are homozygous with respect to the characters in question. It is 

 also clear why the hybrids give rise to hybrids and pure dominants 

 and recessives — half of their gametes bear S, and half bear s. The 

 hybrid plants are heterozygous. 



The real difference then between the F 2 hybrids (Ss) and the 

 pure dominants (SS) is that the former are heterozygous and the 

 latter are homozygous. In order to tell which is which, since they 

 are phenotypically the same, it is necessary to breed them. When 

 self-fertilization can be practiced, as in the case of most plants, 



