Heredity - 489 



Fig. 26-13. Backcross involving two independently asso 

 by which an unknown genotype may be determined. 



of hair color and hair texture in guinea pigs, 

 can also be emphasized from another view- 

 point. By dissociating the color and texture 

 characteristics the F 2 offspring may be classi- 

 fied as follows: 



rough smooth 

 9 + 3 = 



JL + _1 = 



4 = 



black 

 white 



12 



12 



4 



3 



= 3:1 



This shows that the 9:3:3:1 ratio of the F 2 

 generation is obtained merely by superimpos- 

 ing two 3:1 ratios. As to color the ratio is 

 12 blacks to 4 whites; and as to texture the 

 ratio is 12 roughs to 4 smooths. In other 

 words, the guinea pig experiment is just like 

 the case of the garden pea, except that two 

 pairs of genes, instead of one pair, are being 

 transmitted to the offspring, each independ- 

 ently of the other. 



In the organism many allelic pairs of genes 

 are simultaneously undergoing independent 

 assortment as they are transmitted from 

 generation to generation, although only gene 

 pairs that are localized in separate pairs of 

 chromosomes are able to assort independ- 

 ently. In the case of an organism that is 

 heterozygous for three pairs of genes — for 

 example, Aa, Bb, and Cc — eight kinds of 

 gametes: 



A a A a A a A a 

 B b B b b B b B 

 C c c C C c c C 



t'ng pairs of genes. The backcross is the simplest method 



will be formed, provided the allelic genes 

 have loci in different chromosomes. Or, to 

 generalize completely, a heterozygous indi- 

 vidual will form 2" classes of gametes in 

 equal numbers, when n is the number of 

 heterozygous allelic genes with loci in differ- 

 ent chromosomes. 



Variations of Genie Potency. The discus- 

 sion so far has indicated that each gene 

 exerts an "all-or-none" sort of action — but 

 this is not always true. In fact, quite a few 

 cases are known in which a homozygous pair 

 of recessive genes, or a dominant gene 

 whether homozygous or heterozygous, may 

 fail to find any expression in a certain per- 

 centage of the offspring. Such an incomplete 

 penetrance of certain genes increases the diffi- 

 culty of genetic analysis. However, the degree 

 of penetrance may often be increased to ap- 

 proximately 100 percent by suitably adjust- 

 ing environmental conditions during the 

 embryonic period. In such cases the expected 

 Mendelian ratios are obtained among the 

 offspring. 



Variations in expressivity, which is some- 

 what analogous to penetrance, have also been 

 observed in some cases. That is to say, certain 

 genes, whether in heterozygous dominant- 

 recessive or homozygous recessive combina- 

 tion, may show large quantitative variations 

 in the degree to which the phenotypic char- 

 acteristic may appear among the offspring. 

 One recessive gene in Drosophila, for exam- 



