Part 2 (2/2)

When the hybrids mature and their germ cells (eggs or pollen) ripen, each carries only one of these factors, either the red or the white, but not both. In other words, the two factors that have been brought together in the hybrid separate in its germ cells. Half of the egg cells are white bearing, half red bearing. Half of the pollen cells are white bearing, half red bearing. Chance combinations at fertilization give the three cla.s.ses of individuals of the second generation.

[Ill.u.s.tration: FIG. 14. Diagram ill.u.s.trating the history of the factors in the germ cells of the cross shown in Fig. 13.]

The white flowering plants should forever breed true, as in fact they do.

The red flowering plants also breed true. The pink flowering plants, having the same composition as the hybrids of the first generation, should give the same kind of result. They do, indeed, give this result i.e. one white to two pink to one red flowered offspring.

[Ill.u.s.tration: FIG. 15. Diagram ill.u.s.trating a cross between special races of white and black fowls, producing the blue (here gray) Andalusian.]

Another case of the same kind is known to breeders of poultry. One of the most beautiful of the domesticated breeds is known as the Andalusian. It is a slate blue bird shading into blue-black on the neck and back. Breeders know that these blue birds do not breed true but produce white, black, and blue offspring.

[Ill.u.s.tration: FIG. 16. Diagram showing history of germ cells of cross of Fig. 15. The larger circles indicate the color of the birds; their enclosed small circles the nature of the factors in the germ cells of such birds.]

The explanation of the failure to produce a pure race of Andalusians is that they are like the pink flowers of the four o'clock, i.e., they are a hybrid type formed by the meeting of the white and the black germ cells. If the whites produced by the Andalusians are bred to the blacks (both being pure strains), all the offspring will be blue (fig. 15); if these blues are inbred they will give 1 white, to 2 blues, to 1 black. In other words, the factor for white and the factor for black separate in the germ cells of the hybrid Andalusian birds (fig. 16).

[Ill.u.s.tration: FIG. 17. Diagram of Mendel's cross between yellow (dominant) and green (recessive) peas.]

The third case is Mendel's cla.s.sical case of yellow and green peas (fig.

17). He crossed a plant belonging to a race having yellow peas with one having green peas. The hybrid plants had yellow seeds. These hybrids inbred gave three yellows to one green. The explanation (fig. 18) is the same in principle as in the preceding cases. The only difference between them is that the hybrid which contains both the yellow and the green factors is in appearance not intermediate, but like the yellow parent stock. Yellow is said therefore to be dominant and green to be recessive.

[Ill.u.s.tration: FIG. 18. Diagram ill.u.s.trating the history of the factors in the cross shown in Fig. 17.]

Another example where one of the contrasted characters is dominant is shown by the cross of Drosophila with vestigial wings to the wild type with long wings (fig. 19). The F_1 flies have long wings not differing from those of the wild fly, so far as can be observed. When two such flies are inbred there result three long to one vestigial.

[Ill.u.s.tration: FIG. 19. Diagram ill.u.s.trating a cross between a fly (Drosophila ampelophila) with long wings and a mutant fly with vestigial wings.]

The question as to whether a given character is dominant or recessive is a matter of no theoretical importance for the principle of segregation, although from the notoriety given to it one might easily be misled into the erroneous supposition that it was the discovery of this relation that is Mendel's crowning achievement.

Let me ill.u.s.trate by an example in which the hybrid standing between two types overlaps them both. There are two mutant races in our cultures of the fruit fly Drosophila that have dark body color, one called sooty, another which is even blacker, called ebony (fig. 20). Sooty crossed to ebony gives offspring that are intermediate in color. Some of them are so much like sooty that they cannot be distinguished from sooty. At the other extreme some of the hybrids are as dark as the lightest of the ebony flies. If these hybrids are inbred there is a continuous series of individuals, sooties, intermediates and ebonies. Which color here shall we call the dominant? If the ebony, then in the second generation we count three ebonies to one sooty, putting the hybrids with the ebonies. If the dominant is the sooty then we count three sooties to one ebony, putting the hybrids with the sooties. The important fact to find out is whether there actually exist three cla.s.ses in the second generation. This can be ascertained even when, as in this case, there is a perfectly graded series from one end to the other, by testing out individually enough of the flies to show that one-fourth of them never produce any descendants but ebonies, one-fourth never any but sooties, and one-half of them give rise to both ebony and sooty.

[Ill.u.s.tration: FIG. 20. Cross between two allelomorphic races of Drosophila, sooty and ebony, that give a completely graded series in F_2.]

MENDEL'S SECOND DISCOVERY--INDEPENDENT a.s.sORTMENT

Besides his discovery that there are pairs of characters that disjoin, as it were, in the germ cells of the hybrid (law of segregation) Mendel made a second discovery which also has far-reaching consequences. The following case ill.u.s.trates Mendel's second law.

If a pea that is yellow and round is crossed to one that is green and wrinkled (fig. 21), all of the offspring are yellow and round. Inbred, these give 9 yellow round, 3 green round, 3 yellow wrinkled, 1 green wrinkled. All the yellows taken together are to the green as 3:1. All the round taken together are to the wrinkled as three to one; but some of the yellows are now wrinkled and some of the green are now round. There has been a recombination of characters, while at the same time the results, for each pair of characters taken separately, are in accord with Mendel's Law of Segregation, (fig. 22). The second law of Mendel may be called the law of independent a.s.sortment of different character pairs.

[Ill.u.s.tration: FIG. 21. Cross between yellow-round and green-wrinkled peas, giving the 9: 3: 3: 1 ratio in F_2.]

We can, as it were, take the characters of one organism and recombine them with those of a different organism. We can explain this result as due to the a.s.sortment of factors for these characters in the germ cells according to a definite law.

[Ill.u.s.tration: FIG. 22. Diagram to show the history of the factor pairs yellow-green and round-wrinkled of the cross in Fig. 21.]

As a second ill.u.s.tration let me take the cla.s.sic case of the combs of fowls. If a bird with a rose comb is bred to one with a pea comb (fig. 23), the offspring have a comb different from either. It is called a walnut comb. If two such individuals are bred they give 9 walnut, 3 rose, 3 pea, 1 single. This proportion shows that the grandparental types differed in respect to two pairs of characters.

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