488 - Heredity and Evolution 



F, GAMETES 



Fig. 26-12. The P 1 and F ; gametes of a "two-factor cross" involving hair color 

 and hair length in guinea pigs. 



stock produces only one kind of gamete, and 

 all the F, pigs must be alike (Fig. 26-12). But 

 the Fj hybrids produce four kinds of eggs, 

 and a similar four kinds of sperm (Fig. 26- 

 12). Consequently when these gametes unite 

 to form the F 2 generation, the combinations 

 shown at bottom of page 487 are obtained. 



These combinations account for the pheno- 

 type ratio in the F 2 generation, namely: 



black-rough 



9 

 white-rough 



3 



black-smooth 



3 

 white-smooth 



3 



and also for the ratio of the nine genotypes 

 (which are numbered in the Punnett Square): 



®@©©©©©©© 



The random combinations of the Fj eggs 

 and sperm, as indicated by the Punnett 

 Square, shows that four different genotypes, 

 (T), @, ($), and @, are represented in the 

 black-rough phenotype; and this is borne 

 out by backcrossing experiments. Thus, if 

 the unknown is genotype @, the offspring of 

 the backcross will show a 1:1:1:1 ratio of 

 black-rough, black-smooth, white-rough, and 

 white-smooth, as may be seen in Figure 26-13. 

 Similarly genotypes (T), @, and (s) may be 

 determined, if the backcross yields, respec- 

 tively, (1) nothing but black-rough offspring; 

 (2) half and half black-rough and black- 

 smooth; or (3) half and half black-rough and 

 white-rough (as should be worked out by the 

 student). 



The complete independence of each of 

 the two gene pairs, during the inheritance 



