[The frequency of X-linked mental retardation]

600 mentally retarded children, 339 boys and 261 girls aged 5-9 years were examined. Boys to girls ratio (B:G) was found in the total group to be 1.3. Among 257 children with mental retardation of confirmed genetic origin B:G = 1.4, in 129 children with confirmed exogenous defects B:G = 1.04. The significant prevalence of boys over girls was characteristic of children with monogenic forms. The frequency of X-linked mental retardation in the total group, in all mentally retarded boys and in boys with genetic forms was 12.5 +/- 1.3%, 22.1 +/- 2.2% and 28.5 +/- 2.8%, respectively. The frequency of X-linked mental retardation was higher in boys with genetic forms of imbecility.