A cytogenetic study of a population of mentally retarded males with special reference to the marker (X) syndrome.

A cytogenetic survey of a population of 274 mentally retarded males on community placement is described. Thirty-five had an aneuploid chromosome constitution and five had the mar(X) syndrome. The range of clinical variation among the mar(X) probands and their affected relatives is described. Family studies were possible for four of the five mar(X) probands and in two families the mar(X) gene was apparently transmitted through a clinically normal male, suggesting that this type of male transmission may be a not uncommon phenomenon.