GENETICS 



243 



Color in Guinea-pigs. — Among the well known animals, the guinea- 

 pigs furnish excellent material for the study of heredity. There are 

 many color varieties of these animals, which breed true; that is, whenever 

 one member of a pure breed is mated to another like itself, the offspring 

 are all like their parents. Uniformly black animals constitute such a 

 true-breeding variety (Fig. 190). Albinos form another such variety 



Fig. 190. Fig. 191. 



Fig. 190. — A black, smooth-coated guinea-pig. 



Fig. 191. — An albino smooth-coated guinea-pig. {Both figures hy courtesy of Professor 

 W. E. Castle and the Harvard University Press.) 



(Fig. 191). Albinos are animals in which the usual pigments of the 

 skin, hair and eyes are wanting. As a result the body is entirely white, 

 and the iris of the eye pink. Two albinos mated together always pro- 

 duce albino children. 



If, now, a black guinea-pig of a pure breed is mated with an albino 

 guinea-pig, the offspring will all be black. Ordinarily the black offspring 



FiG. 192. — Diagram showing the results,' througl«f|rwo hybrid generations, of mating a 

 pure black and an albino guinea-pig. 



are indistinguishable from the black parent. But when these hybrid 

 blacks are mated together, the two original colors reappear. Some of 

 the members of the second hybrid generation are black, others are albino. 

 Roughly the blacks constitute three-fourths of the total, the albinos one- 

 fourth. 



These results are stated in tabular form in Fig. 192, in which the 

 parents are referred to as Pi, their offspring as Fi (meaning first filial 



