Heredity - 485 



R 



gametes 



R 



gametes 



75% 



25% 



Fig. 26-9. Cross between yellow- (GG) and green- (gg) seeded garden peas. 

 Due to the dominance of the "yellow gene" (G), heterozygous (Gg) seeds dis- 

 play the same appearance as homozygous (GG) ones. 



(Fig. 26-9). All the F, yellow seeds belong 

 to the same phenotype (a type of organism 

 judged on the basis of its appearance), but 

 two different genotypes (types based on the 

 genie constitution) are represented. Some of 

 the yellow F 2 seeds (Fig. 26-9) are homo- 

 zygous (GG) but others are (Gg), or hetero- 

 zygous; that is, the two genes of an allelic 

 pair are not the same. 



A dominant-recessive relation between 

 allelic genes is the general rule rather than 

 the exception among animals and plants, al- 

 though frequently the dominance is not al- 



together complete. When the dominance is 

 complete, as in the present case, the presence 

 of a single recessive gene cannot be deter- 

 mined by the appearance of its possessor, 

 but only by the breeding potentialities. If 

 the recessive gene is present, the correspond- 

 ing trait will turn up in a subsequent genera- 

 tion, as soon as the homozygous condition 

 happens to be established at the conception 

 of some future individual. In other words, 

 although organisms of the same phenotype 

 may look alike, they do not breed alike, 

 unless they also belong to the same genotype. 



