Page:Popular Science Monthly Volume 67.djvu/213

Rh cross-breeding between the two, followed by selection for pure individuals, will within two generations give the desired combination of characters in individuals which will breed true. This process of selection is simplest when the characters to be combined are recessive in nature, but individual breeding-tests become necessary when dominant characters are included in the combination desired.

If a character gives blending inheritance, it must be treated in a different way. Suppose, for example, that we desire to combine lop-ears in rabbits with albinism, but that our lop-eared stock consists wholly of pigmented animals. How shall we proceed? First, mate a pigmented lop-(Fig. 1) with a short-eared albino (Fig. 2). The offspring will be pigmented half-lops (Fig. 3). If two of these be bred together their young will all be half-lops, and about one in four of them will be albinos. Now these albino half-lops may be mated with pure pigmented lops. The young will again all be pigmented, but will this time be three-quarter lops, and by breeding these together albino three-quarter lops may be obtained in the next generation. By continuing this process of back-crossing with the lop-eared stock, and selecting the albino offspring obtained, the lop-eared character may be steadily improved in the albinos until it is practically as good as in the original lop-eared stock. The rate of improvement possible can be readily calculated. The albino young will be:

This will be the result on the hypothesis that no secondary variation occurs in the lop-eared character. If, however, variation is induced by the cross-breeding, then it is possible that the desired end may be reached sooner, or that an even better lop may be obtained in the albino cross-breds, than that of the original pigmented stock.

Latent characters are an important element in practical breeding. Sometimes they greatly aid the breeder's work; sometimes they impede it. If a stock contains undesirable latent characters which are brought into activity by cross-breeding, these latent characters will have to be eliminated, or a new stock tried.

Since cross-breeding is likely to modify characters even when these conform to the laws of alternative inheritance, and is certain to modify them when they give blended inheritance, it should be practised with extreme caution, and only by the breeder who has a definite end in view and a fairly clear idea of how he is going to attain it.

The purity of standard breeds should be carefully guarded, and