The principles of heredity with some applications / by G. Archdall Reid.

  • Reid, G. Archdall (George Archdall O'Brien), Sir, 1860-1929.
Date:
1906
Catalogue details

Licence: In copyright

Credit: The principles of heredity with some applications / by G. Archdall Reid. Source: Wellcome Collection.

    390/406 (page 368)
    (i. e. an albino), or lias an extra digit, or lacks a phalanx, then the inheritance tends to be alternative. We have constantly- spoken as if alternative inheritance were a rare phenomenon. In the sense meant the statement was true, and has deceived no one. But in another sense it is not true. In all species, the members of which are divided into males and females, all the larger differences between the mating individuals are subject to alter- native inheritance. The male sexual characters are latent in the females, the female characters are latent in the males. It has been surmised that the inheritance of the sexual characters is Mendelian. It is very much more probable the inheritance of Mendelian characters is sexual. Nature, especially in the case of the more complex plants and animals, has devised all sorts of methods to secure cross-fertiliza- tion, the most effectual being complete sexual differentiation. All very complex types, therefore, present two methods of inheritance, the blended and the alternative. Smaller parental differences are subject to the former, larger differences to the latter. In blended inheritance there is of necessity no latency, even though one individual be more or less completely prepotent over the other. In alternative inheritance there is constant latency, but no true prepotency, though there may be something which is often confused with it but is radically different, namely, dominance1 ; for dominance involves latency of the alternative character, which prepotency does not This dual method of inheritance provides the machinery whereby a maximum amount of regression is secured. Evolution, no matter how stringent the selection from which it has resulted and by which it is maintained, is never perfect. Often as regards the sexual characters, primary and secondary, but more especially as regards the latter, we see, not complete dominance and latency as in perfect alternative inheritance, but some degree of blending. This occurs in its most pronounced form in hermaphrodites. More common is a slighter degree of blending, as when women display rudimentary moustaches, or men unusually large mammae. Distinct from this blending, though perhaps generally combined or occurring with it, is the transference of a male or female character from the set in which it is normally found to the opposite set—as when women have long thick beards or men fully-developed female breasts. Nature has evolved alternative inheritance to create and per- petuate sexual differentiation, but she is not a consciously discriminating agency, and, just as the blending of sexual characters sometimes occurs, so, on the other hand, the inherit- ance of non-sexual characters is sometimes alternative. As we have seen, whenever the latter happens the non-sexual differences are, like the sexual differences, usually considerable. Nature 1 See Reports to Evolution Committee, I, p. 137.