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.

    47/144 page 31
    HEREDITY AND ENVIRONMENT 31 It seems that genes whose action is easily influenced by the environment are found in all groups of animals and plants which have been the subject of genetic research. Two examples out of many may be cited from the work on Drosophila melanogaster, as illus¬ trating different types of environmental effects. A single recessive factor causes the reduplication of one or more of the legs. Sometimes only a few joints are affected, but at others all the legs may be doubled even down to the base. This factor only produces an effect at low temperatures. When the flies are reared in an ice chest, normal segregation is obtained in crosses with the wild type, but at higher tempera¬ tures, say 23° C., it is quite without effect even in the homozygous condition (Höge, 1915). Similarly, ' abnor¬ mal abdomen a semi-dominant sex-linked factor, results in a more or less complete obliteration of the regular banding of the abdomen. This character is fully expressed in cultures obtained from freshly made- up stock bottles. It becomes less pronounced in the flies which hatch later, until, in old cultures, all are completely normal, even the homozygous females. The abnormal effects are, however, once more obtained in full when the offspring of such phenotj^ically normal but genotypically abnormal flies are reared in a fresh culture medium. It has been found that moisture is the necessary agent for evoking the action of this gene ; when the cultures dry up it can operate no longer. Both changes in temperature and in the acidity of the culture media have been excluded as causative agents (Morgan, 1915). Such instances as these might be greatly multiplied. Among plants a striking example of the interaction of genetic factors with the environment is that de¬ scribed by Baur. At a temperature of 15°-20° C.,
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