Mendel's Hypothesis, Designed to Explain the Phenomena of Heredity.—Mendel's great service is to have conceived a hypothesis that seems to have disclosed the key adapted to unlock all the secrets of heredity.
While the body of an individual is the resultant of forces so mutually exclusive that the appearance of one characteristic means the disappearance of its antagonist; in the development of the sexual cells the two antagonistic characters are distributed in equal proportion. That is to say, one-half of the male cells contain the dominant character, and one-half the recessive; and the same holds true for the female cells. The characters of the two parents, in other words, never merge in the reproductive cells, but are distributed in equal measure, independently of the question whether they are dominant or recessive. Thus for example: in the case already cited of the first hybrid generation of the peas with red flowers, in every one of the plants, without distinction, half the pollen has potentially the red character and half has the white; and in the same way the female cells have, half of them a red potentiality and half of them a white. Such hybrids of the first generation, therefore, although apparently similar to the parent with red flowers, differ in their germinative powers, which are not made apparent in the individual. And the same may be said of hybrid nettles with indented leaves, etc.
Granting Mendel's hypothesis, we have on the one hand pollen and on the other seed ready to come together in every manner included within the range of possible combinations; the individual is, in its characteristics, nothing else than the product of a combination which must necessarily manifest itself in accordance with the well-known mathematical laws of probability.
For instance, let us proceed to diagram the possible disposition of the sexual cells of the hybrids of peas, all of them having red flowers. In terms of percentage, they will give, out of every hundred, fifty red and fifty white.
P = pollen; O = ova; R = red, dominant; w = white, recessive:
The possible number of combinations between the pollen grains and the ova are four; namely, RR, Rw, wR, ww. But where a dominant characteristic encounters a recessive (Rw, wR), the recessive disappears, to make way in the individual for the dominant characteristic alone. The definitive result is three individuals of dominant character, to one of recessive character.
Fig. 6.
Nevertheless, the hybrids of dominant character are not all equal among themselves. Those belonging to the combination RR, indeed, are permanent in character and in all respects alike, and they reproduce the original red-flower progenitor. The other red-flower hybrids, belonging to the groups Rw and wR are, on the contrary, similar to the hybrids of the first generation and contain reproductive cells differentiated in character; such hybrids, if reciprocally fertilised, will again give three dominant offspring to every one recessive; that is, they will obey the law of disjunction. The hybrids belonging to the fourth group, on the contrary, are constant, like those of the first group, and are permanently of recessive character; and they will reproduce the original progenitor with white flowers.
The same results may be attained with nettles with smooth and indented leaves, and with all other types of plant and animal life that obey the laws of Mendelism.