Mendelism and evolution / by E.B. Ford.

  • Ford, E. B. (Edmund Briscoe), 1901-1988.
Date:
1931
Catalogue details

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

Credit: Mendelism and evolution / by E.B. Ford. Source: Wellcome Collection.

    55/144 page 39
    HEREDITY AND ENVIRONMENT 39 whole may be slight, but all must combine to form a gene-complex providing an internal inherited environ¬ ment varying from individual to individual. In such an environment, then, every gene has to act. As it is variable, it is evident that the effect of a given gene may be changed, even in a constant external environ¬ ment, but not the gene itself. This principle is responsible for a phenomenon very common in genetic work, that of the improvement in viability of a new mutation after several generations of inbreeding. Those individuals having a gene- complex with which the gene in question reacts in a particularly disadvantageous way are removed by selection. Of a similar nature are such instances as that worked out by Timoféefï-Ressovsky (19276) on ' radius incompletus ' in Drosophila funebris. This character is due to a single factor ; yet it was possible to establish true-breeding stocks in each of which its expression differed, owing to its reaction with a selected gene-complex. The integrative action of genetic factors may, how¬ ever, be demonstrated more clearly by the behaviour of such a gene as that producing the ' eyeless ' char¬ acter in Drosophila melanogaster. This is a recessive carried in the fourth chromosomes. Its effects are variable, but the eyes are considerably smaller than normal and they may be absent. By selection it has been possible to produce a stock in which most of the flies are without eyes. If, however, an unselected stock, homozygous for the eyeless factor, be inbred for several generations, practically all develop normal eyes. Further inbreeding shows that the modification which has been produced is permanent, and is not the result of temporary changes in the external environ- ment.
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