[Footnote 17: The Production of Hybrids in the Vegetable Kingdom.]
Mendel's discoveries with regard to peas and the influence of heredity on them, were founded on very simple, but very interesting, observations. He found that if peas of different colors were taken, that is to say, if, for instance, yellow-colored peas were crossed with green, the resulting pea seeds were, in the great majority of cases, of yellow color. If the yellow-colored peas obtained from such crossing were planted and allowed to be fertilized only by pollen from plants raised from similar seeds, the succeeding generation, however, did not give all yellow peas, but a definite number of yellow and a definite number of green. In other words, while there might have been expected a permanence of the yellow color, there was really a reversion in a number of the plants apparently to the type of the grandparent. Mendel tried the same experiment with seeds of different shape. Certain peas are rounded and certain others are wrinkled. When these were crossed, the next generation [{209}] consisted of wrinkled peas, but the next succeeding generation presented a definite number of round peas besides the wrinkled ones, and so on as before. He next bred peas with regard to other single qualities, such as the color of the seed coat, the inflation or constriction of the pod, as to the coloring of the pod, as to the distribution of the flowers along the stem, as to the length of the stem, finding always, no matter what the quality tested, the laws of heredity he had formulated always held true.
What he thus discovered he formulated somewhat as follows: In the case of each of the crosses the hybrid character, that is, the quality of the resultant seed, resembles one of the parental forms so closely that the other escapes observation completely or cannot be detected with certainty. This quality thus impressed on the next generation, Mendel called the dominant quality. As, however, the reversion of a definite proportion of the peas in the third generation to that quality of the original parent which did not appear in the second generation was found to occur, thus showing that, though it cannot be detected, it is present, Mendel called it the recessive quality. He did not find transitional forms in any of his experiments, but constantly observed that when plants were bred with regard to two special qualities, one of those qualities became dominant in the resultant hybrid, and the other became recessive, that is, present though latent and ready to produce its effects upon a definite proportion of the succeeding generation.
Remembering, then, that Mendel means by hybrid the result of the crossing of two distinct species, his significant discovery has been stated thus: The hybrid, whatever its own character, produces ripe germ cells, which bear only the pure character of one parent or the other. Thus, when one parent has the character "A," in peas, for example, a green color, and the other the character "B," in peas once more a yellow color, the hybrid will have in cases of simple dominance the character "AB" or "BA," but with the second quality in either case not noticeable. Whatever the character of the hybrid may be, that is to say, to revert to the example of the peas, whether it be green or yellow, its germ cells when mature will bear either the character "A" (green), or the character "B" (yellow), but not both.
As Professor Castle says: "This perfectly simple principle is known as the law of segregation, or the law of the purity of the germ cells. It bids fair to prove as fundamental to a right understanding of the facts of heredity as is the law of definite proportions in chemistry. From it follow many important consequences."
To follow this acute observer's work still further by letting the crossbreds fertilize themselves, Mendel raised a third generation. In this generation were individuals which showed the dominant character and also individuals which presented the recessive character. Such an observation had of course been made in a good many instances before.
But Mendel noted--and this is the essence of the new discovery in his observations--that in this third generation the numerical proportion of dominants to recessives is in the average of a series of cases approximately constant--being, in fact, as three to one. With almost absolute regularity this proportion was maintained in every case of crossing of pairs of characters, quite opposed to one another, in his pea plants. In the first generation, raised from his crossbreds, or, as he calls them, hybrids, there were seventy-five per cent dominants and twenty-five per cent recessives.