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.

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    makes the mistake, so to speak, of treating them as sexual. Of course, however, the alternative inheritance of large non-sexual differences is not certain, not so “ clean ” as that of sexual differences, the alternative inheritance of which has been established by a long course of selection. It nearly approaches perfection in the case of haemophilia, which, when it occurs, is almost as much a male character as a beard. But in most instances anomalies occur which tend to link up the perfectly alternative type of inheritance with the perfectly blended type. Thus, as in the case of Mr. Hurst’s poultry hybrids, partial blending was very common. Again, the alternation of Mendelian characters is not so perfect as that of the sexual characters; one of a pair is generally so dominant over the other that when the two are brought together, the latter is almost always reduced to latency—for example, when an albino is crossed with a normal individual, the offspring are generally all pigmented, the albino only reappearing, if at all, in subsequent generations.1 Lastly, the various items of a set of Mendelian characters do not tend to hang together with the persistency which usually characterizes the sexual characters. Unlike the latter they behave as discrete allelomorphs, each of which is independently capable of displacing its opposite number. This is only what might be expected ; it has been a main “ object ” of Natural Selection to evolve a definite combination of sexual characters, but not of Mendelian characters. A circumstance which tends to confuse our thinking on this subject, and so to obscure our recognition of the essential simi- larity between the Mendelian and the sexual phenomena, is the followingWhen we cross two types that possess alternative Mendelian characters, the first generation consists of impure dominants. If these are self-fertilized or bred together, as they usually are in Mendelian experiments, we get pure dominants, impure dominants, and pure recessives, in more or less definite proportions. The pure dominants can then be bred together, 1 It should be noted that, in man at least, the dominance of the pigmented type over the albino is by no means complete. Suppose an individual varies so from his parents as to be a pure albino. The chances are he will mate with a pigmented individual. His children will be pigmented impure dominants. The allelomorphs will separate in the gametes. Albinos are so few that they practically always mate with pigmented persons. Therefore, if the dominance of the pigmented type were complete, all the offspring and descendants should be pigmented, except in the very rare cases when an albino allelomorph meets another of the same kind. But the descendants of albinos are so frequently albinos, that the latter hypothesis seems excluded. It appears clear, therefore, that the albino descendant of an albino is probably not always a pure recessive, but that he is often an impure dominant in whom the albino character has the upper hand. In this imperfect dominance of the pig- mented type, albinism resembles the sexual characters which are alternatively dominant. B B