226 



PRINCIPLES OF ANIMAL BIOLOGY 



generation is known as the Fi generation (abbreviated from the words 

 first filial). The white coat is not lost in the Fi animals, however, for 

 when they are mated together they produce an F2 (second filial) gener- 

 ation consisting of some blacks and some whites. In a large collection 



Fig. 193. — Black and white guinea pigs, with smooth coats. (Courtesy ofProfessor W. E. 



Castle and the Harvard University Press.) 



of such F2 families the black animals are found to make up about three- 

 fourths of the total number, the wliites about one-fourth. 



These results are explained by the diagram in Fig. 194, where the 

 genes involved are symbolized by letters — the white gene by w, the 

 black gene by W. The two letters under each parent are its genetic 



formula, the single letter under these 



White 



WW 



w 



Eggs' 



Black X 

 WW 



W 



\ 



F, V/w (Block) 



w>=^— ^ w 



Sperm 



the formula of the germ cells of that 

 parent. The Fi generation has the 

 formula Ww, and the animals are 

 black because one gene W is capable 

 of producing black pigment just as 

 well as two IF's are. That ability 

 of one gene to do the work of two is 

 what is called dominance; W has 

 that ability, w does not. 



When the reduction division 

 occurs in Fi animals, two kinds of 

 germ cells are produced because the 

 two genes are different. Some eggs 

 and spermatozoa contain W, some 

 contain w; and the numbers of the 

 two kinds are about equal. When two kinds of eggs, equally numerous, 

 are fertilized at random by two kinds of spermatozoa, equally numerous, 

 four combinations result, also about equally numerous. These combi- 

 nations are WW, Ww, wW, and ww, as shown in the F2 line of the figure. 

 The first three of these are black, the last one white — hence the 3:1 

 ratio of blacks and whites. The two middle formulas are identical, and 



MWW /4Ww MwW A WW 



Black Black Black White 



Fig. 194. — Inheritance of black and 

 white color in guinea pigs: W, gene for 

 black; w, gene for white. 



