Genetic studies in mental subnormality.

Date:
1973
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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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    DISCUSSION 27 4. Dermatoglyphicfindings As discussed in Chapter V comparisons of the dermatoglyphic findings in males and in females cannot be made. The mean total finger ridge count obtained for males in families with only males affected was lower than that in families with both males and females affected, and the mean total a-b ridge counts were significantly different. These findings suggest that there are distinct differences in families with only males affected as compared with families where there are females affected. Summary of the evidence of a contribution by X-linked genes to idiopathic severe subnormality The evidence compatible with a contribution from X-linked genes in idiopathic severe subnormality may be summarized as follows : (a) there is an excess of males, (b) there are a remarkable number of pedigrees compatible with X-linked inheritance, (c) the data appear compatible with a hypothesis that there are in the families ascertained two types of sibship : one where males effectively only are at risk, and one where males and females are equally at risk, (d) the data do not suggest compatibility with a hypothesis whereby inheritance is by a multifactorial/threshold mechanism with the higher threshold in females, (e) further support for a difference between the two types of family was obtained from dermatoglyphic investigations; in all- male affected families the total finger ridge counts and total a-b ridge counts in affected males are lower than in affected males in families with females affected also. In the writer's opinion these observations suggest that there is a contribution to idiopathic «evere subnormality by X-linked recessive genes. Due to the mode of ascertainment of cases the data could not be used to give any quantitative estimate of this contribution. Although a contribution from this source cannot account for all the excess of males among the severely retarded, it may well be substantial. It has to be remembered that when an effectively lethal X-linked recessive gene is segregating in a population, most of the families with an affected individual will contain only a single affected male. This is because in any generation, one- third of the affected males will have received a freshly arisen mutated gene, i.e. they are mutants. In the remaining families where the gene has been passed by a heterozygous woman, in many cases only one male in a sibship will be affected, i.e. he is a single segregant, and this is especially true where the average male sibship size is not large. For instance, if heterozygous women have two sons, only one in four of them would be expected to have both sons affected. These elementary considerations illustrate some of the difficulties which arise when attempting to demonstrate a contribution to aetiology from X-linked recessive genes, since severe subnormality is frequently not associated with other distinctive clinical abnormalities. References Âkesson, H. O. (1961). Epidemiology and genetics of mental deficiency in a southern Swedish population. Almqvist and Wiksells. Uppsala. Allan, W., Herndon, C. N. and Dudley, F. G. (1944). 'Some examples of the inheritance of mental defi¬ ciency : apparently sex-linked idiocy and micro¬ cephaly.' Amer. J. ment. Defic., 48, 325-34. Alter, M. and Brühl, H. H. (1967). 'Dermatoglyphics in idiopathic mental retardation.' Amer. J. Dis. Child., 113, 702-6. Bell, J. (1940). 'A determination of the consanguinity rate in the general hospital population of England and Wales.' Ann. Eugen., 10, 370-91. Berg, J. M. (1961). In: Proceedings of the second international congress on Mental Retardation. Vienna. Basel: Karger. Borgaonkar, D. S. (1968). 'Dermatoglyphics in partial G trisomies.' Lancet, i, 1428. Campbell, M. (1965). 'Gauses of malformations of the heart.' Brit. med. J., a, 895-904. Carson, N. A. J. and Neill, D. W. (1962). 'Metabolic abnormalities detected in a survey of mentally backward individuals in Northern Ireland.' Arch. Dis. Childh., 37, 505-13. Davison, B. G. G. (1967). 'Concordance and discordance of congenital heart disease in 20 families.' J. med. Genet., 4, 245-50. Dewey, W. J., Barrai, I., Morton, N. E. and Mi, M. P. (1965). 'Recessive genes in severe mental defect.' Amer. J. hum. Genet., 17, 237-56. Dunn, H. G., Renpenning, H., Gerrard, J. W., Miller, J. R., Tabata, T. and Federoff, S. (1963). 'Mental retardation as a sex-linked defect.' Amer. J. ment. Defic., 67, 827-48.