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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    PENETRANCE AND EXPRESSION unpigmented areas, increases moderately with increasing age of the mother. The penetrance of genetic factors predisposing to Polydactyly, in the same animal, undergoes marked reduction with increasing maternal age ; in some genotypes, the incidence of the abnormality is five times as high among the progeny of mothers aged 3-6 months as it is in litters produced when they are a year older. The penetrance of hereditary hare-lip in the mouse also decreases with increasing maternal age. In man, no effect of maternal age on the incidence of hare-lip or cleft palate is discernible. On the other hand, in a series of 582 cases of severe congenital malformations of varied types, in some of which genetic factors are with little question aetiologically important, Murphy (1947) found convincing evidence for the significance of maternal age—severe malformations, in the aggregate, appear to be three or four times as common among births to mothers in their forties as they are two decades earlier ; specific types of malformation would doubtless reveal considerably greater differentials. Mongoloid idiocy, for which there is some reason to suspect genetic predisposi¬ tion as an aetiological factor, occurs among children born to mothers in their late forties with forty or fifty times the frequency found among children of mothers in their twenties (Penrose, 1949). In congenital pyloric stenosis, which may depend upon the presence of a single autosomal gene with about 20 per cent penetrance in homozygous condition (Cockayne and Penrose, 1943) the effects of maternal age may be in the opposite direction. The apparent excess of cases among first-born children has frequently been commented on and the reality of the excess has been estabUshed by Ford, Ross and Brown (1941). These ob¬ servations are usually interpreted as indicating that conditions specifically asso¬ ciated with primogeniture are responsible for the relatively high incidence among first-born children ; but the data strongly suggest that there is a fairly consistent decline in frequency with increasing birth rank, so that the intra-uterine conditions favouring manifestation should perhaps be sought among those which change with maternal age (or with successive pregnancies) rather than in conditions peculiar to primogeniture. Variations in temperature during developmental stages have been found to have conspicuous effects on the manifestation of several score of genes in Drosophila. It is unfortunate that there appear to have been no phenogenetic studies in mammals in which experimental alteration of maternal body temperature during pregnancy was attempted ; it has been shown experimentally, however, that the localized development of pigment (after birth) in the pelage of Himalayan (C^'/C^) rabbits and in Siamese cats is temperature dependent. In poultry, Sturkie (1943) has studied the eff'ects of temperature reduction in varying degree and at different times during incubation on the manifestation of a gene for Polydactyly. Among control stocks, the gene was fully penetrant in homozygous condition, and about 94 per cent penetrant in heterozygotes ; with the most effective treatments, penetrance was roughly halved in the homozygotes, and in heterozygotes was reduced to only 4 per cent. Nutrition Eff'ects of nutrition on gene expression have long been known. Yellow fat is formed in rabbits of genotype Y ¡Y, when xanthophyll is present in the diet ; feeding a diet free of xanthophyll suppresses this manifestation of the Y gene, 12