Page:Popular Science Monthly Volume 80.djvu/380

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This is written DD and RR, not simply D and R, because we are supposing that each individual is pure for the character involved, that is, has received D or R from each parent.

First filial generation DR DR DE DE, as many as there may happen to be. These are written DE because each gets D from one parent (which has nothing else to give) and of course E from the other. Now in the next generation each parent contributes, not its whole "D," but one or the other, according to the laws of chance. Accordingly, DR DR may produce a DD, or a DR, or a RR, and as a matter of fact, they do so. Why should there be any particular numerical proportion? If we put black and white balls in a bag, and draw them out in pairs at random, the chances are equal that we shall get two alike, or two different. It is so with our crosses. The cases in which we get two alike may be of two kinds, both black or both white, or in the case of the crosses, both D or both R. The cases in which we get two different are necessarily alike, black and white, or D with R. Hence, according to the law of chance, we expect in the third generation the following:

1. Both alike, DD and RR.
2. Not alike, DR and RD, which are the same.

Now we have seen that because of dominance R does not show when D is present, so that a DR looks like a DD. Consequently, of the above four cases, three show the dominant character, and one (RR) shows the recessive. The whole diagram may now be reconstructed:

1. DD RR (original cross).

2. DR DR DR DR (first filial generation).

3. DD DR RD X RR (second filial generation, or grandchildren). How can this be confirmed? Obviously, if the facts are as here given, the DD and the RR of the third line are now pure, in spite of the fact that the DD had an RR grandparent and a DR parent, and the RR a similarly complicated ancestry. Take a number of these pure types, now called "extracted recessives" and "extracted dominants," and breed them separately, the DDs with DDs, and the RRs with RRs, and they will breed true, and their descendents will forever remain true, unless contaminated by a cross, or some new variation arises. The DRs, however, when bred together, will again produce the "three-to-one" results, just like their parents. Consequently, it is possible to extract a pure strain out of an impure one, a fact of tremendous scientific and practical importance.

Mendel's results were published in Brünn in 1866, but attracted little or no attention. They never became known to Darwin, who would have immediately perceived their importance. In 1884, when Mendel died, no one had the slightest idea that his name would ever be familiar to scientific workers, though Mendel himself used to say "Meine Zeit