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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    PATTERNS OF DISTRIBUTION 11 but a woman who was borderline subnormal/ severely subnormal had three illegitimate children, two of whom were subnormal/severely subnormal. In addition to these five families, there was another family (No. 85) where an educationally subnormal woman who had a subnormal sister, had two illegitimate affected children, one of whom was the offspring of an incestuous union with her brother. It is in these families where there are subnormal parents of severely subnormal children that multifactorial inheritance seems the most likely explanation. As the objective of this paper has been pri¬ marily to consider the possible contribution of X-linked genes to uncharacterized severe sub- normality, consideration of families where there is strong evidence against an X-linked explana¬ tion has been superficial. However, it will be clear that many of these families with affected individuals in different sibships are not readily explained by any unifactorial hypothesis. On the other hand, as already pointed out, although milder degrees of subnormality would be expected to show marked familial aggregations on any multifactorial hypothesis, perhaps it would not have been expected that so many families would be ascertained where the multiple cases are of severe subnormality. The bizarre patterns of distribution of cases in these families underline the need for care in attributing a specific hypothesis to those where the pattern strongly suggests autosomal recessive inheritance or, for that matter, in all-male affected families to assume X-linked inheritance without reservation. The data present many difficulties. When dealing with families with males and females, or females only affected, there is an additional complication in that the overall frequency of males, whether familial or non-familial cases, is higher than that of females. This phenomenon is, however, taken into account during later analyses (Chapter IV). Table V may be found convenient for rapid consultation. Fuller data are given in Appendix II, and from these more sophisticated analyses may be undertaken if desired. Data on the sex composition of sibships are ignored in this table but are considered in Chapter IV. Consanguinity in families where members were severely subnormal It has long been known that in any large series of parents of severely defective individuals, the frequency of consanguinity is higher than in the general population. Data on the frequency of consanguinity for Great Britain as a whole, or for any specific area, are scanty. Bell (1940) estimated that the frequency of first cousin marriages in the parents of patients admitted to general hospitals was о-8 per cent. However, the frequency is probably lower now, judging by questioning of over 3,000 families at genetics clinics and in course of ad hoc studies of unifactorial traits in the Oxford area. It is difficult to imagine that the frequency of cousin marriages of parents of people resident in the Oxford area is as high as о • 5 per cent. In the present study, reliable information was obtained in respect of parents of patients in 93 families, and in eight (8-6 per cent) the parents were related in some degree. In addition, in two families severely subnormal females had incestuous unions producing children who did Table V Distribution of idiopathic severe subnormality in families with only females or with both males and females affected Types of sibships in families where only females or both males and females were affected No. of sibships No. of individuals Affected Unaffected Total Only one sibship per family with affected individuals. . . . . . . . .. .. 38 94 84 178