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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    CHAPTER I Introduction It has long been known that among mental defectives there is an excess of males and a number of authors have pointed out that this excess increases with the degree of defect. The causes of mental defect are numerous and the clinical pictures are various. However, for some purposes it is convenient to divide cases into two main groups. Into the first can be placed those cases where the underlying environmental or genetic mechanisms are understood and/or the clinical picture is a recurring one. Into this group will fall cases due to single gene segrega¬ tions and chromosomal abnormalities which are identified by characteristic clinical or biochemical signs. It will also include cases due to identifiable adverse prenatal and postnatal environmental factors and to perinatal anoxia or trauma. In individual cases, of course, the causes may not be recognized. An increasing number of single gene traits, including those due to autosomal recessive and X-linked recessive genes and chromosomally determined traits, have been recognized in recent years ; but apart from mongolism, most of them are uncommon and probably determine only a small percentage of all mental retardation in children. The effects of single segregating genes and chromosomal abnormalities are almost invariably severe. The second group may be regarded as consisting of mentally retarded individuals where the cause is not known and the clinical picture is not recognized as part of a specific syndrome. These range in severity from mild to severe, and although most cases are sporadic, familial cases do occur. In this monograph these cases will be referred to as 'idiopathic' or 'mental retardation of unknown aetiology'. It is generally assumed that the great majority of less severe idiopathic mental retardation cases are of the 'subcultural' type and that the cause of most of these is multifactorial. Evidence from correlations of intelligence test scores between affected individuals and their first-degree relatives is compatible with the hypothesis that a high proportion of intellectual variation is determined predominantly by the effect of multiple additive genes. However, there is an excess of severely defective individuals of the idiopathic type over expectation on an additive multifactorial hypothesis. An undue proportion of such severely affected patients have affected relatives and Penrose (1938) has shown that the parent/child correlations for intelligence are low. This is hardly surprising as such severely affected individuals rarely reproduce and the familial incidence is predominantly contributed by affected sibs. There are indications of raised parental consanguinity rates in families where more than one sib has severe idiopathic mental defect, which suggests strongly that there is a contribution from autosomal recessive genes whose expression is not recognizable as specific either clinically or biochemically. In those families with members having idiopathic mental defect there is an overall excess of affected males and it is not surprising that from time to time it has been suggested that some part at least of this male excess could be due to X-linked recessive genes whose expression, as that of autosomal recessive genes, is not currently identifiable or specific. A number of authors have reported individual families where there are only affected males distributed in a pattern compatible with X-linked recessive or sex-limited autosomal dominant inheritance. However, evidence of this type is always difficult to interpret, for there are a number of possible explanations for the occurrence of males distributed in families in such a manner. Nevertheless, the occurrence of such families and the increasing excess of males among the severely defective, suggested that a I