Nonspecific X‐linked mental retardation I: A review with information from 24 new families

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Abstract
Clinical manifestations and other aspects of nonspecific X‐linked mental retardation are reviewed using data from the literature and information on affected males in 24 new families ascertained in British Columbia. A great degree of variability was apparent in the mental abilities of affected males. Speech defects, other CNS disorders and minor physical changes such as “big” ears or a highly arched palate were not present in many cases. Evidence for the existence of a clinical entity of mental retardation associated with the fragile site at Xq27 or 28 and macro‐orchidism is discussed. Genetic phenomena of reduced penetrance in males and of partial expression in females with respect to X‐linked recessive genes are examined. Consideration is given to the question of whether this type of mental retardation is due to X‐linked recessive or autosomal dominant sex‐limited genes. Most ascertained cases of X‐linked mental retardation are from families of northern European extraction. Recommendations are made regarding the diagnosis and counseling of X‐linked mental retardation cases.