MENDEL'S LAW OF HEREDITY 



which differ simultaneously in two or more 

 independent unit-characters. Crossing thembe- 

 comes an active agency for the production of 

 new varieties. 



In discussing the crosses now to be described 

 it will be convenient to refer to the various 

 generations in more precise terms, as Bateson 

 has done. The generation of the animals orig- 

 inally crossed will be called the parental gen- 

 eration (P) ; the subsequent generations will 

 be called filial generations, viz. the first filial 

 generation (F x ), second filial (F 2 ), and so on. 



When guinea-pigs are crossed of pure races 

 which differ simultaneously in two unit-charac- 

 ters, the Fj offspring are all alike, but the F 2 

 offspring are of four sorts. Thus, when a 

 smooth dark animal (Fig. 22) is crossed with 

 a rough white one (Fig. 23) the Fj offspring 

 are all rough and dark (Fig. 24), manifesting 

 the two dominant unit-characters, dark coat 

 derived from one parent, rough coat derived 

 from the other. But the F 2 offspring are of 

 four sorts, viz. (1) smooth and dark, like one 

 grandparent, (2) rough and white, like the 

 other grandparent, (3) rough and dark, like 



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