IT is generally recognized to-day that the central idea of Mendel's discovery in regard to heredity is that when two contrasting elements enter a hybrid, one from each parent, they separate in the germ cells of the hybrid, so that the germ-cells are pure like those of the original parents in regard to each element. Chance meetings of the germ cells give the ratios that are characteristic of Mendelian heredity. This is illustrated by the example that Mendel gave.
When a pea having green seeds is crossed with a pea having yellow seeds a hybrid pea is produced. When the germ-cells of the hybrid are ripened, each ovule carries either the element for green or that for yellow, but never both. Yellow and green have separated. The same separation occurs in the formation of the pollen. If self-fertilization now takes place chance combinations of the yellow-or green-bearing ovules with the yellow or green-bearing pollen give one pure yellow pea, two hybrid (yellow) peas, one pure green pea, as shown in Fig. 1. Mendel discovered that the same principle holds when two pairs of
- Mendel speaks of characters as forming pairs. To-day we speak of factors or genes as the paired, elements (allelomorphs) in the germ-cells, and these are supposed to act as differentials in producing the characters in the adult animal or plant. The English school considers the presence of a factor as one allelomorph and its absence as the contrasting factor. For instance, if yellow color is due to a present factor then if it were lost the color that results is green. But since we know nothing about the material in the germ-plasm that by interacting with other parts gives yellow in one case and green in the other, it seems to me gratuitous to postulate the nature of the change in the germ plasma. It is only necessary to assume that the original factor and a new factor form a pair without in any way committing one's self as to how these two allelomorphic factors are related to each other.