Pedagogical Anthropology, Maria Montessori [online e book reader TXT] 📗

Fig. 5.

That is, the characteristics which belonged to the first two parents all survive, even though in a latent form, in the descendants; and they continue to differentiate themselves in well established proportions. In one offspring out of four, the characteristics of the grandfather, which have remained dormant in the father, once more reappear. This intermittent heredity of characteristics, that are passed from grandfather to grandson, overleaping the father, is one of the best-known laws of pathological heredity in man; and it is called atavistic heredity, to distinguish it from direct heredity, which denotes the transmission from parent to offspring. But no explanation had ever been found for this sort of phenomenon. Undoubtedly, it must be connected with the phenomena of Mendelism.

Accordingly, in the second generation Mendel's second law has been established, the law of disjunction, which is stated as follows:

Mendel's Second Law: "In the second generation obtained by reciprocal fertilisation of the first hybrids, three quarters of the offspring will exhibit the dominant character, and one quarter the recessive."

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.