Mendel's Law
as stated by him, refers to qualities, not to individuals. If, for instance, a tall red pea be crossed with a short blue pea, reverting to the table, we should have as before 496 red flowers and 496 blue flowers. We should have 496 pure long stems and 496 pure dwarf stems in the fifth generation, but half of the pure long stems would bear blue flowers, and half of the pure short stems would bear red flowers.
Nor is this the end of the matter. Red and length are dominant qualities. They are stronger than blue and a dwarf habit. The hybrids have long stems and red or reddish flowers. Still another consideration: we may find pure stems having hybrid flowers and hybrid stems bearing flowers of a pure color. A long stem and a red flower are not a guaranty of pure qualities. A long stem and a blue blossom are a guaranty of pure color only. A dwarf stem and a red flower are a guaranty of stem only. A dwarf stem and a blue flower will breed true every time. By keeping the strains separate, we may obtain a long stem and blue flower, and a dwarf stem and a red flower that breed true. In time the hybrids will run out, and we shall have the two old types with which we started and the two new combinations of length and color. Now if we admit the shape of the pod, and have a long-stemmed, red-flowered, short-podded plant crossed with a short-stemmed, blue-flowered, long-podded plant, we shall have as many long pods as short pods in the fifth generation, but the number of plants having all three characteristics of either one of the ancestors is reduced again.
The probability of an offspring having all of a dozen characteristics of an ancestor becomes an interesting problem in mathematics. About all that is worth saying in this connection is that the plant that has bred back to blue flowers will produce only offspring having blue flowers; that is to say, the hybrid or medium color, size, length of stem, etc., tends to fade out. Starting with a red flower and a long stem and a blue flower and a short stem, it is not difficult to obtain a plant having a long stem and a blue flower that will produce its like. It is not difficult to breed a plant having a short stem and a red flower that will breed its like; but it is difficult to breed plants that have a medium length and are constant. Nature does not favor hybrid sizes, shapes, or colors.
Mendel's law applies to animals as well as to plants. Dr. E. C. Schroeder of the United States Department of Agriculture has carried on experiments in crossing animals at the Bethesda station in the outskirts of Washington. He chose a rat all gray and a white rat with a black hood. His experiments have been continued for many generations. Tier after tier of rat cages are labeled with pedigrees. As expected, gray has proved the dominant color, and hooded white the recessive color. Of the first generation resulting from this cross every member was solidly gray like the dominant strain of the parents. This was as Mendel said it would be. Members of this generation were crossed. In this case the gray rats of the first generation produced part gray and part hooded. The hooded rat that had failed to make itself felt in the first generation showed in twenty-five per cent of the second. These hooded rats bred hooded in the following generations. A portion of the grays, twenty-five per cent, bred all grays, and a remaining proportion still having the unset characteristics repeating the proportions of the second generation. The experiment is going on still, and figures are being kept, but the length of it is already so great as to leave no doubt as to the findings, and they agree with those of the Austrian monk. Mendel's law is correct.
See Mutation