Chap. 20 THE PHYSICAL BASIS OF HEREDITY 401 



from one another in two pairs of contrasting characters. A male guinea pig 

 that has a pure-line ancestry for short, black hair is crossed with a female 

 pure-line for long, white hair, or vice versa for sex (Fig. 20.11). The genes 

 in the body cells are dominant black (WW) and dominant short (LL) that is 

 WWLL in the male, and recessive white (vv-vv) and recessive long (//) in the 

 female, wwll. During meiosis the genes on homologous pairs of chromosomes, 

 i.e., WW, or LL, or ww or //, go to different sperm or eggs as the case may 

 be (Fig. 20.11). 



These gametes form the offspring of the Fi generation, all of them black 

 short haired guinea pigs {WLwl) having the dominant genes for black and 

 short {W and L) in their body cells as well as the recessive ones for white 

 and long (vv and /). The gametes of the Fi generation will contain the genes 

 WL, Wl, wL, wl (Fig. 20.1 1 ). If animals of the Fj generation are intercrossed 

 the ratio of their offspring will be: 9 black short, 3 black long, 3 white short, 

 1 white long. The combinations of genes in the eggs and sperm that produce 

 these are shown in Figure 20.11, with the combinations of genes in the body 

 cells. Since in each sex there are four kinds of gametes, there will be 16 pos- 

 sible combinations of gametes with their contained genes in the animals of 

 the Fo generation. 



The foregoing experiment shows that whenever strains of animals differ 

 from one another in two or more pairs of genes the inheritance of one pair is 

 independent of the other (Mendel's Law of Independent Assortment). 



W'Black 

 W" White 



Black Short 

 Chtterozy^ous) 



L'Short 



WhUtLonj 



tA Black hBlack %.\/hxtc ^Whik 



*" Short "*Lorvg Short ''Loncf 



Fig. 20.10. Back- (or test) cross, a common method of testing the gene content 

 of animals that look alike but may differ genetically. A backcross of a black, 

 short-haired guinea pig to a white, long-haired one shows that the black, short- 

 haired guinea pig carried genes for white color and long hair, i.e., the animal did 

 not breed true to type. (Courtesy, Winchester: Genetics. Boston, Houghton 

 Mifflin Co., 1951.) 



