92 GENETICS AND EUGENICS 



cise terms, as Bateson has done. The generation of the 

 animals originally crossed will be called the parental genera- 

 tion (P); the subsequent generations will be called fihal 

 generations, viz., the first filial generation (Fi), second fihal 

 (F2), and so on. 



When guinea-pigs are crossed of pure races which differ 

 simultaneously in two unit-characters, the Fi offspring are 

 all alike, but the F2 offspring are of four sorts. Thus, when 

 a, smooth colored animal (Fig. 31) is crossed with a rough 

 albino (Fig. 32), the Fi offspring are all rough and colored 

 (Fig. 33) , manifesting the two dominant unit-characters, — 

 colored coat derived from one parent, rough coat derived 

 from the other. But the F2 offspring are of four sorts, viz., 

 (1) smooth and colored, like one grandparent, (2) rough and 

 albino, like the other grandparent, (3) rough and colored, 

 like the Fi generation, and (4) smooth and albino, a new 

 variety (Fig. 34). It will be seen that the pigmentation of 

 the coat has no relation to its smoothness. The dark animals 

 are either rough or smooth, and so are the white ones. Pig- 

 mentation of the coat is evidently a unit-character indepen- 

 dent of hair direction, and as new combinations of these two 

 units the cross has produced two new varieties, — the rough 

 colored and the smooth albino. 



Again, hair-length is a unit-character independent of hair- 

 color. For if a short-haired colored animal (either self or 

 spotted, Fig. 35) be crossed with a long-haired albino (Fig. 

 36), the Fi offspring are all short-haired and colored, but 

 the F2 offspring are of four sorts, viz., (1) colored and short- 

 haired, like one grandparent, (2) albino and long-haired, 

 like the other, (3) colored and long-haired, a new combina- 

 tion (Fig. 37), and (4) albino and short-haired, a second new 

 combination (Fig. 38). 



Now the four sorts of individuals obtained from such a 

 cross as this will not be equally numerous. As we noticed in 

 connection with the simple cross of colored with albino 

 gumea-pigs, dominant individuals are to the corresponding 

 recessives as three to one. Therefore, we shall expect the 



