486 - Heredity and Evolution 



In comparing the experiments so far, gar- 

 den peas give exactly the same results as 

 Andalusians, if one determines the F, ratios 

 in terms of genotypes rather than in terms of 

 the phenotypes. The ratio is 1:2:1, with 

 reference to genotypes (BB), (Bb), and (bb) 

 in the Andalusian; and also 1:2:1 for geno- 

 types (GG), (Gg), and (gg) in the garden pea 

 (Figs. 26-7 and 2li- ( J). The different pheno- 

 type ratios — 1:2:1 for Andalusians, and 3:1 

 for the peas — merely indicate that among 



Andalusians, each different phenotype rep- 

 resents a distinct genotype; whereas among 

 garden peas, the yellow phenotype, due to 

 dominance, consists of two genotypes. 



Backcrossing to Determine Genotype. The sim- 

 plest method of determining the genotypes 

 present in a given phenotype is to backcross 

 the unknown F, individuals. This procedure 

 involves mating the unknown to its homo- 

 zygous recessive grandparent (Fig. 26-10). 

 The advantage of the backcross is that it gives 



Fig. 26-10. The backcross, i.e., mating 

 an unknown individual to an individual 

 that is homozygous-recessive for the 

 genes in question. If the unknown is 

 homozygous for the dominant gene 

 (top), only one kind of offspring is ob- 

 tained; if heterozygous (bottom), two 

 kinds are produced. 



50% 



50% 



