Heredity - 487 



very simple results, which are easy to analyze 

 — as may be seen in the following experi- 

 ment. 



Any yellow-seeded F 2 pea (Fig. 26-9), 

 which may be either homozygous (GG) or 

 heterozygous (Gg), is backcrossed to a green- 

 seeded (gg) plant and the offspring are ex- 

 amined. If the unknown is of the (GG) geno- 

 type, all the offspring will have yellow seeds, 

 as is shown in Figure 26-9 (the F, genera- 

 tion). But if the unknown is genotype (Gg), 

 only half the offspring will be yellow and the 

 other half will be green, as is shown in Fig- 

 ure 26-10. Thus the backcross gives a simple 

 1 : 1 ratio, if one deals with a heterozygous 

 individual, as compared to the single product 

 obtained from a homozygous specimen. 



Dominance does not in any way alter the 

 mechanics of transmission of a gene, but 

 merely changes the potency of the gene as a 

 determinant of developmental processes in 

 the organism. But how the genes, which are 

 transmitted via the zygote to all the cells of 

 the body, manage to produce their profound 

 effects upon developmental processes is a 

 problem that will be discussed later (p. 529). 



Independent Assortment: Two or More 

 Gene Pairs in Separate Chromosome Pairs. 

 During gametogenesis several or many pairs 

 of chromosomes are "dealt out" simultane- 

 ously to the maturing gametes; and conse- 

 quently it is impossible to predict the exact 

 hereditary constitution of any given egg or 

 sperm. But two or three gene pairs, even if 

 localized in different pairs of chromosomes, 

 can be followed. This permits the hereditary 

 results to be predicted very accurately, as in 

 the following experiment on the inheritance 

 of hair characteristics in guinea pigs. 



The experiment starts with two well- 

 known laboratory stocks of the guinea pig. 

 One stock with black, smooth hair (Fig. 26- 

 11) has been inbred for many generations 

 without any change in the color and texture 

 of the coat; and the same is true of the other 

 stock, which has white hair and a rough coat. 

 When crossed, these stocks produce nothing 

 but black-rough individuals in the Fj genera- 



Fig. 26-11. Cross between a black-smooth and a 

 white-rough guinea pig. The F, individuals are all 

 black-rough; but when inbred, these produce four kinds 

 of offspring in the proportions: 9 black-rough, 3 black- 

 smooth, 3 white-rough, and 1 white-smooth. 



tion. But when inbred, the ¥ 1 pigs give F 3 

 offspring that are black-rough, black-smooth, 

 white-rough, and white-smooth, respectively, 

 in a ratio of 9:3:3:1. 



On the basis of the known history, the 

 genotype of the Pj, black-smooth stock must 

 be (WW ss) — that is, homozygous for both 

 the "black gene" (IF) and for the "smooth 

 gene" (s); and the genotype of the white- 

 rough pigs must be (ww SS) — that is, homo- 

 zygous for both the white (w) and rough 

 (S) genes (Fig. 26-12). Consequently each Pj 



