Mendel's Law
Mendel's Law names 2 different subjects across the 6 encyclopedias on this shelf. Each is set out below.
Mendel's Law as stated by him
Aiton's Encyclopedia (1910)
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
Mendel's Law a law of hybrids
Aiton's Encyclopedia (1910)
a law of hybrids. It is named from its discoverer, Johann Mendel, an Austrian monk, Abbot of Brunn, who lived 1822-1884. He carried on experiments in cross-pollenation of peas in the garden of the monastery. His observations were published in an obscure Brunn journal and were overlooked for a time. His chief conclusion, stated in an elementary way, is this: If the pollen from a red flower be used to fertilize the ovules of a blue flower, and the seed thus produced be planted, one-fourth of the plants will bear red flowers; one-fourth will bear blue flowers and one-half will bear hybrid blossoms, or blossoms of mingled red and blue. Now, if the seeds of the hybrid blossoms be planted,--we might naturally expect hybrid blossoms; but here is where Mendel's law really comes in; one-fourth of the plants thus produced will bear red flowers; one-fourth blue flowers, and one-half will bear hybrid flowers. Seeds of the red and of the blue flowers produce plants bearing red and blue flowers respectively, but the hybrids tend to split up into pure bred forms. The tendency may be expressed in a table. Let us suppose that each seed produces a plant, and that each plant bears a single flower producing four seeds. If we plant the seeds of a single hybrid flower, and continue to plant all the seeds for five generations, we shall have flowers as follows: Red Hybrid Blue Total First planting . . . . . 1 2 1 4 Second planting . . . . . 6 4 6 16 Third planting . . . . . 28 8 28 64 Fourth planting . . . . . 120 16 120 256 Fifth planting . . . . . 496 32 496 1024 The hybrids split into reds and blues, so that they only double in each generation; while the reds and blues not only quadruple, but they gain from the hybrids. The hybrids are one-half of the first crop, but they are only one-thirty-second of the fifth crop, and in a few more generations they become an insignificant part of the whole. The conclusion is that pure types tend to run out hybrids; or put in another way, hybrids tend to go back--to split up into pure strains. Another portion of Mendel's law may be illustrated without a formal statement. If we are studying a hybrid of red and blue, and red be a strong, decided characteristic while the blue is of a delicate quality, red will predominate in the hybrid and three-fourths of the flowers of the first planting will be red or dominated by red, while the more delicate blue is characteristic of one-fourth only. In such case, blue is indicative of pure blood, while we cannot tell from color alone and without planting whether a red flower be a pure red or a hybrid. In such cases, red is called a dominant quality; blue is a recessive quality. Mendel carried on observations covering many points other than the colors of petals. He observed differences in the form of ripe seeds,--wrinkled, smooth; color of seed albumen--yellow, orange, green; color of seed coat--gray, white, brown, spotted; form of ripe pod--inflated or compressed between seeds; color of unripe pod--light green, dark green, yellow; position of flowers; length of stem--nine inches to seven feet; etc.
What Has Changed in the Last Century
Aiton's Encyclopedia in 1910 had the essentials right "Mendel's law is correct" but described the heterozygotes as bearing "blossoms of mingled red and blue," which is blending rather than the dominance it goes on to describe; in Mendel's peas the first cross is uniformly dominant, giving the familiar 3:1 appearance and 1:2:1 constitution. Since then the invisible "qualities" have been given a physical home: T. H. Morgan's fruit-fly work from about 1910 tied them to chromosomes and showed that characters on the same chromosome are linked, so Aiton's confident "interesting problem in mathematics" of independent assortment holds only for genes far apart. Wilhelm Johannsen had just coined the word "gene" in 1909, and after Watson and Crick's 1953 structure of DNA a Mendelian factor became a readable sequence of bases; single-gene human diseases such as cystic fibrosis are now traced to specific mutations. Mendel himself, called "Johann" here, is now universally known by his religious name Gregor, and his 1866 paper is treated as the founding document of genetics rather than an obscure Brunn curiosity.
Written for this edition, 2026.