Clinical genetics / edited by Arnold Sorsby.

Date:
1953
Catalogue details

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

Credit: Clinical genetics / edited by Arnold Sorsby. Source: Wellcome Collection.

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    EXPRESSION although Y ¡Y (and Yjy) rabbits can still be distinguished from those not possessing the У-allele by the fact that there is no xanthophyllase in their livers. In poultry, the manifestation of a dominant gene for yellow shanks is similarly contingent upon the presence of Carotinoid pigments in the food. A striking illustration of the effects of a dietary supplement on the expression of a gene-contingent patho¬ logical trait in a mammal is seen in the results of administering massive doses of vitamin A to mice homozygous for the rhino (hypotrichosis cystica) gene (Fraser, 1949). Untreated mice of this genotype, after shedding the first pelage, remain hairless throughout life ; there is marked hyperkeratosis of the hair follicles, and a progressive development of cysts in the follicles and in the sebaceous glands. Vitamin A treatment fails to restore the hair coat, and apparently it will not cause a regression of cysts that have already formed ; but it does inhibit cyst development, so that old mice which have been treated from youth with the vitamin preserve a relatively normal skin. It is of interest to note that the rhino condition in mice has points of resemblance to human hyperkeratosis foUicularis (Darier's disease), which appears in some families to be contingent on the presence of a single dominant gene, and which has been reported sometimes to respond favourably to vitamin A therapy. Conceivably, the hyperkeratosis gene may act by increasing the vitamin A demands of the affected epithelial tissues. (That individual differences in nutri¬ tional requirements can be genetically determined has been shown in experimental animals in numerous instances : in poultry, for example, with respect to riboflavine demands, by Lamoreux and Hütt (1948) ; and in rats, with respect to choline requirements, by Engel (1943).) Infection Genetic differences in respect to susceptibility to infection of course have obvious manifestations only when there is exposure to the appropriate infecting organism. Differences of this kind have been amply demonstrated in experimental animals (for review, see Gowen, 1948) and there is little doubt of their existence in man, for example in the case of tuberculosis (Kallmann and Reisner, 1943) or of diphtheria (Roshng, 1929). Analogously, genetic predisposition to allergic disease (Wiener, Zieve and Fries, 1936) requires contact with an appropriate allergen for full manifestation of the genes involved. Hormone levels Hormone levels in many cases conspicuously influence the expression of specific genotypes. In so far as the nature or quantity of the hormone concerned is itself determined by genes other than those responsible for the effect which the hormone modifies, such cases might formally be classed as illustrative of the effects of genetic modifiers {see below) ; but since hormone levels are rather generally subject to environmental modification, these cases also illustrate the effects of environmental agencies on gene expression. The most thoroughly studied example is pituitary dwarfism in the mouse. Mice homozygous for the dw gene fail to grow appreciably after the second or third week of life ; they remain sexually infantile, and are completely sterile. Histological examination reveals pathology of the anterior pituitary and abnormalities in other endocrine organs which are strikingly similar to those found in hypophysectomized rats. An almost complete clinical cure of these mice can be produced by the daily implantation 13
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