Page:Popular Science Monthly Volume 67.djvu/207

Rh contain, unseen, the recessive character. Otherwise we may keep getting mixed lots of offspring. The simplest and surest way of making such a test is to mate the dominant animal with a recessive. For in that case, if the dominant is pure, all the offspring will be dominants; but if the dominant parent is a hybrid, half the offspring will be recessives, half dominants. When the purity of two dominant animals of opposite sex has once been established by breeding-test, we may use them as the starting point of a race of dominants which we may be sure will breed true.

We must not, however, fall into the error of supposing that any pair of dominants which produces only dominant offspring is, therefore, pure. For progeny of this sort will be obtained if only one of the parents is pure, the other being hybrid. In starting a race of dominants which will breed true we must test each animal individually, by mating it, preferably with a recessive, or else with a dominant known to be hybrid in character. A test of the former sort should, as stated, give 50 per cent, of recessive individuals if the dominant is impure; a test of the latter sort should give 25 per cent, of recessives, if the dominant is impure. Either sort of test should give only dominant offspring if the dominant tested is pure.

The statement has already been made that many characters are independent of one another-in heredity; T hope now to demonstrate the correctness of this idea m cases of alternative inheritance, even when the independent characters relate to the same bodily parts. For this purpose the coat-characters of guinea-pigs and rabbits are well adapted, since they are exterior structures easily studied in the living animal. I hope to show first that pigmentation of the hair is inherited quite independently of its length, and secondly that hair-arrangement (in smooth or rough coat) is inherited quite independently of both pigmentation and length of hair.

When an ordinary short-haired guinea-pig (Fig. 5) is mated with a long-haired albino guinea-pig (Fig. 7), all the young produced are short-haired and pigmented, these being the dominant characters. (See Fig. 8.) Xow if the cross-bred young are bred together, offspring of four different sorts are produced. Two of the four sorts are identical with the grandparents in character; they are short-haired pigmented animals (Fig. 5) and long-haired albinos (Fig. 7), respectively. But the other two sorts represent new combinations of characters; they are short-haired albinos (Fig. 9), and long-haired pigmented animals (Fig. 10). Further, these four sorts of individuals occur on the average in definite numerical proportions, viz: