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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    IO GENETIC STUDIES IN MENTAL SUBNORMALITY multifactorial or segregating modifiers and in individual families such an explanation seems on occasion to be plausible. 3. Affected members of the family could simply have received a poor endowment of relevant multiple genetic factors, and have experienced environmental influences which determined that they fell towards the lower tail of the distribution of intelligence quotients. Only a minority of cases of severe subnormality could be accounted for in this way. Estimates can be made of the frequency in a population of all mental defectives below intelligence quotient of 50 who can be assigned to a normal distribution of intelligence, and these estimates can then be compared with the observed frequency; the difference between the observed and expected frequencies would represent other than subcultural cases. It is not possible to make an estimate of the expected proportion of subcultural cases in families as ascertained in the present study. It has been estimated (Fraser Roberts, 1952) that over 15 times as many individuals are observed with intelligence quotients below 50 as would be expected if all variation in intelligence quotient levels was normally distributed. Almost certainly a smaller proportion of familial cases are so caused. The explanation is more plausible if one parent, although not severely subnormal, is subnormal. 4. It is possible also to advance a multi- factorial/threshold hypothesis. Such an explana¬ tion has similar implications for family patterns as a purely multifactorial hypothesis. The 91 families can conveniently be allotted to 5 groups: {a) cases occur in one sibship (38), (b) cases occur in the same generation but in more than one sibship (3), (c) cases occur in two or more generations (15), {d) cases born to a mother who was sub¬ normal (6), (e) families where pedigree information is not reliable (29). Pedigrees of some of these families are shown in Fig. 2. (a) Severe idiopathic subnormality in one sibship There were 38 families where severe sub- normality in two or more members occurred in only one sibship. In two of these families (Nos. 9 and 198) the parents were known to be con¬ sanguineous but the significance of this will be discussed later. In five families (Nos. 2, 33, 61, 87 and 179) affected members had similar physical abnormalities which also will be considered later. (¿) Severe idiopathic subnormality in the same generation but in more than one sibship In three families the affected members were in more than one sibship but in the same generation. In family No. 3, II i was illegitimate, severely subnormal and had three affected half sibs. Both affected members in family No. 208 were illegitimate. They had the same mother but one was the offspring of a brother/sister mating. In the remaining family, No. 214, a woman had two illegitimate affected children by two different men and then married a sub¬ normal man who had an affected female half sib. [c) Severe idiopathic subnormality in more than one generation There were 15 families where severe sub- normality occurred in more than one generation and the mothers of affected individuals were not themselves subnormal (Nos. 7, 8, 30, 36, 40, 56, 80, 82, 109, 129, 149, 152, 194, 195 and 196). None of the affected individuals in the same family had any physical abnormality in common, {d) Severe idiopathic subnormality in offspring of subnormal women There were five families where a subnormal woman had at least one severely subnormal child (Nos. II, 16, 47, 147 and 198). In family No. 11, a woman who was subnormal had three illegitimate children, of whom only one was of normal intelligence. In family No. 16, three females were affected. Ill 11 had an affected daughter and an affected niece. The only case examined by the author in family No. 147, IV 6, was the offspring of a brother/sister mating. Both her parents were at least subnormal and they had several sibs who were affected. A subnormal woman in family No. 198 was the offspring of a first cousin marriage and she had a severely subnormal son. Tkere was little information available for family No. 47