Page:Popular Science Monthly Volume 62.djvu/276

270 discussion it is to be understood, unless otherwise stated, that the parent forms are all well-established varieties or species (i.e., they come true to type from seed), and that the hybrids are close fertilized (either self-fertilized or fertilized by others like them).

Mendel's law is, briefly stated, as follows: In the second and later generations of a hybrid, every possible combination of the parent characters occurs, and each combination appears in a definite proportion of the individuals.

Mendel did not leave his work unfinished. He proposed a theory, or rather deduced one by a simple course of reasoning that renders the theory almost an established fact as far as results thus far secured are concerned, that explains the facts in the case in a brilliant manner. As I have shown elsewhere, this theory explains most of the so-called exceptions to the law pointed out by several investigators. It explains so many apparent anomalies in such a simple manner that there is danger of applying it too extensively. This point will be brought up again below.

The theory may be illustrated as follows: Suppose the two parent varieties differ with respect to a single character. For instance, a variety of bearded wheat is crossed with one that is smooth (not bearded). When the hybrid thus produced forms its germ cells (pollen and ovules), the characters of the two parents separate, the beard-producing character passing into part of the pollen grains and part of the ovules, the smooth-head character of the other parent passing into the remaining germ cells. The character passing into any single germ cell being governed by chance, on the average half the pollen and half the ovules will receive the character of one parent, the other half that of the other. The results of this separation of characters may be illustrated by the following diagram, in which B stands for the beard-producing character and 8 for its opposite. The reason for the use of a small b in designating the hybrid will appear later.

Since the ovules of each kind are offered both kinds of pollen, half of each, on the average, will be fertilized by one and half by the other kind of pollen, giving the four fertilizations shown at the right. (Abundant experience has shown that S X B=B X S.) From this it appears that one fourth of the progeny of the hybrid Sb will be like the parent B, one fourth like S, and one half like the hybrid itself.