Page:Popular Science Monthly Volume 62.djvu/283

Rh of its progeny would then be, supposing the primed characters to be recessive:

The total number of types occurring in the progeny is 3″; the number of fixed types (homozygote) is 2″. The number of types with r latent characters is nCr2 n-r, where nCr is the number of combinations of n things taken r at a time. Only one type, namely, ''A′B′C′. . . N′,'' consisting entirely of recessive characters, could be selected out without getting with it one or more heterozygote types. But by saving the seed of each tree of this generation separately, and observing which, with close fertilization, would reproduce true to type, we could at once secure, in fixed from, all the 2 homozygote types.

If the tree happened to be of the type VgSb, discussed above, in which all the characters of its original parents are present, the above process of analysis would give, not only all its original parents, but all possible combinations of them, and each in a form that would reproduce true to seed, if self-fertilized. If it were of the type VgB, in which some of the parent characters are missing, it would give all the original parents whose characters are all present, together with all their combinations with characters that are present from other parents, some of whose characters are missing. Since apples are confessedly many times multihybrids, it is probable that a very large number of seed would have to be used to secure all the types capable of resulting from combinations of the N pairs of characters.

Suppose we neglect all but essential characters; we might, in the case of the apple, reduce the number of types to a point which would make the task a possible one. If this were done, it would mean much to the plant breeder. Having a large number of fixed varieties of apples, supposing, of course, that Mendel's law holds, we could select parents with a view to producing any combination of characters we desire. Did not Downing, many years ago, assert that much better results could be secured in producing new seedling apples by using seed from strains that had already been propagated several generations from seed? And why? Possibly because the continuous propagation from seed tends to produce pure strains. If the seed used were always produced by self-pollination, there certainly would be a tendency to pure strains if Mendel's law applies. This problem is worth working out, both from practical and from theoretical grounds. It could be done more easily with strawberries, or with some of the common ornamentals that do not reproduce true to seed. This method of analysis is one way of testing Mendel's law in such groups.

This subject is too new to permit of any useful generalizations