Animal genetics / Frederick B. Hutt.

  • Frederick Hutt
Date:
[1964]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Animal genetics / Frederick B. Hutt. Source: Wellcome Collection.

    423/578 page 401
    CHANGES IN GENES AND CHROMOSOMES 401 determining color of the aleurone layer of the seed, had one effect when the male gamete carrying it came from a homozygote, but a different effect when that same allele came ( in a male gamete ) from an R'R^' heterozygote. In the latter case, most of the pollen carrying R'^ produced effects like those of Presumably had been influenced by during their association in the heterozygote, a hitherto unknown phenomenon to which Brink gave the name paramutation. When we think of a mutation as a change, we might well ask: A change from what? The Drosophila geneticist usually thinks of mutations from the wild type, but is familiar with those occurring within lines that are themselves mutations from the wild type. With domestic animals, for some of which the wild type is not well known, it is easier to think of mutations as changes from what is considered normal for the breed, strain, or species. For example, the spontane¬ ous appearance of polled animals among Dorset Horn sheep in North Carolina, and in Australian Merinos, resulted from dominant muta¬ tions. If, in polled breeds of sheep like Suffolks, Southdowns, and others, horned animals should appear and their condition be proven hereditary, they too could be charged to a mutation, in that case probably recessive. The fact that both the polled sheep in the Dorsets and Merinos and the horned ones in the polled breeds might thus result from mutations at one locus is consistent with the evi¬ dence from other organisms that mutations are not always in the same direction of change. Changes from the normal to the mutant type are best known, but reverse mutations, from the mutant type back to the normal one, also occur. If we consider that the polled breeds of sheep undoubtedly arose by preservation in them of muta¬ tions from the horned state of their wild ancestors, then, if in some polled breed the horned condition reappeared by mutation, that could be attributed to reverse mutation, or, as it is sometimes called, to hack mutation. One should remember also that any advantageous mutations are likely to be preserved in wild animals, and thus become normal for that species. When domestication occurs, the breeders select fur¬ ther among any mutations observed to preserve those considered desirable. Mutations that would be deleterious in the wild state thus become not only normal for some breeds of domesticated ani¬ mals, but also indispensable witnesses to the purity of their breed¬ ing. This is amply attested by the completely albinotic New Zealand White rabbits, polled cattle and sheep, Pekingese dogs, and such bizarre breeds of the fowl as the Frizzle and the Silkie (Fig, 18-1).
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