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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    2 genetic studies in mental subnormality Study of the familial incidence by sex of indivi¬ duals with severe idiopathic mental defect might yield evidence which could test a hypothesis for a contribution by X-linked recessive genes. The Literature Findings in studies of hospital and other populations of mental defectives Penrose (1938), Malzberg (1953), Âkesson (1961), Dewey, Barrai, Morton and Mi (1965) and other authors have noted an excess of male subnormality both in hospital and non- institutionalized patients. All have recognized that an undue proportion of these severely affected defectives have either affected sibs or other affected close relatives. Unfortunately, the majority of authors restricted an examination of the sex ratio and the familial incidence to patients below an arbitrary selected level of intelligence. They either ignored what was known of the clinical picture or of the aetio- logical factors concerned; or they removed arbitrarily from the data which they used for analysis only a few types of cases very obviously due to specific factors. The result was that the case material was not screened clinically with enough care to isolate a clearly defined 'idio¬ pathic' group. No serious attempt had been made to classify mental defect by specificity of clinical features and familial incidence until Penrose (1938) reported his findings in 1,280 institutionalized mental defectives and their families. All his patients were examined and grouped by age and sex, and an attempt was made to attribute the underlying cause of mental defect in each individual. When all known causes had been excluded, the residual group amounted to about a quarter of the total. However, the excluded group included all patients with epilepsy which was not associated with any physical abnor¬ mality and those who were really psychopaths, and so this residual idiopathic group would possibly have contributed a different proportion of the total if other criteria had been used. There is considerable difficulty in getting good estimates of the proportion of patients with severe mental defect which would be classified as idiopathic. For example. Berg (1961) examined 800 severely subnormal patients admitted to hospital over a ten-year period and concluded that as high a proportion as two-thirds could not be attributed to specific aetiological factors or distinct syndromes. Reports of individual families where severe mental deficiency was thought to be inherited as an X-linked recessive trait A number of families have been reported where idiopathic mental defect was inherited in a pattern compatible with X-linkage. Martin and Bell (1943) suggested such an explanation for a family where only males were affected. However, in this family there were two normal males who had normal daughters, who, in turn, had affected sons, so some failure of penetrance had to be postulated to meet this hypothesis. A family described by Losowsky (1961) included ten affected males and two affected females in four generations. The females were said to be less severely affected than the males and the author suggested some degree of dominance of a segregating X-linked gene. The family described by Allan, Herndon and Dudley (1944) can hardly be considered as having idiopathic mental defect since many of the affected males had muscular dystrophy. Dunn, Renpenning, Gerrard, Miller, Tabata and Federoff (1963) described a family with severe idiopathic mental defect inherited in a manner compatible with X-linkage. However, in this family there were other affected members where mental defect was of known aetiology, including phenylketonuria and Down's syndrome. Kerr (1965) noted several families with idiopathic defect where the inheritance was compatible with X-linkage, but the mental defect in different families varied from mild to severe abnor¬ mality. The family reported in the literature where X-linkage was most strongly suggested by pedigree evidence and the mental deficiency was both idiopathic and severe, was one described by Renpenning, Gerrard, Zaleski and Tabata (1962). In this family males in several genera¬ tions were affected and their mothers were of normal intelligence.