Genetic studies in mental subnormality.

Date:
1973
Catalogue details

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

Credit: Genetic studies in mental subnormality. Source: Wellcome Collection.

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    introduction: the literature 3 Findings in surveys of various populations of mental defectives where authors considered the possibility of an X-linked contribution RosanofF (1931) studied a series of pairs of twins, each ascertained by one severely defective member, and noted that there was a marked excess of affected males in the unlike sex pairs. He suggested a contribution by X-linked genes as at least a partial explanation for this excess but he made no analysis to test the hypothesis. Sjögren (1932) also suggested that the excess of affected males with severe defect could be due to X-linked recessive genes but, again, he did not attempt any critical analysis from his studies of populations of mental defectives. Penrose (1938) did not consider that there was any evidence for a contribution to the male excess from X-linked recessive genes. Dewey, Barrai, Morton and Mi ( 1965) compared data obtained from ascertainment of families with more than one case of mental defect living in Wisconsin with those of families with both single and mul¬ tiple cases as ascertained by Penrose (1938). They excluded hydrocephalics, mongols and any cases which they felt were due to exogenous factors, but there must have been included in the residuum many cases attributable to specific genetic factors. They analysed sibships with two or more affected members and found that there was a high inbreeding coefficient in these families. In their opinion the excess of males in families with both single and multiple cases could neither be attributable to X-linkage nor to increased susceptibility of males to exogenous factors. However, they do not record the relevant data. Studies where familial cases were ascertained and then classified according to whether only males, only females or both sexes were affected There appear to have been only three investi¬ gations carried out where familial cases were ascertained and the families then subsequently grouped, depending on whether only cases of one or both sexes were affected. All three studies included patients of both mild and severe defec^ exceeding even the arbitrary intelligence quotient limit of 70 which was specified in the papers. Wright, Tarjan and Eyer (1959) identified 61 families where twO' or more sibs were retarded. However, they included as affected some sibs with intelligence quotients over 80 and in their series there were cases of both known and unknown aetiology. Insufficient data were presented to permit analysis along the lines followed later in this paper. Priest, Thuline, LaVeck and Jarvis (1961) gave few clinical details about their patients and did not specify sexes of cases of known aetiology. They ascertained 83 families but they did not classify clearly the ranges of levels of intelligence of the patients that they were considering. Wortis, Pollack and Wortis^ (1966) included in their survey of 30 families^ cases not only of mental defect but of mental disturbance. Although all the above authors reported a marked excess of families where all males were affected over those where all females were affected (in all instances it was at least 2 to i), none did more than speculate as to the possibility of an X-linked contribution. Only Wright et al. (1959) found a family in the series where the pattern of inheritance was compatible with X-linkage and in this family the affected males were probably suffering from a syndrome causing both ocular and neurological changes. What emerges from the literature, then, is a general agreement that there is an excess of males ; that this excess of males is more marked the more severe the degree of mental defect; and that there is probably an undue proportion of families where only males are affected. How¬ ever, when considering a sign or symptom so very heterogeneous in origin as mental defect, it seems essential to resolve the heterogeneity of aetiology as far as possible before seriously considering the hypothesis that there is a contribution by X-linked recessive genes to^ severe mental defect.