OBJECTIVES: 1) Determine the clinical features of forms of non-specific X-linked mental retardation, especially that associated with a fragile X chromosome, develop methods of diagnosis by cytogenetic studies affected males and heterozygous carriers and of prenatal diagnosis, and identify possible metabolic and endocrine defects. 2) Determine the frequency of chromosome abnormalities, especially those detectable only by high resolution banding, in mentally retarded persons. 3) Study the effects of 13/14 Robertsonian translocations on the reproductive capacities of balanced carriers. 4) Determine the specific phenotypic effects of unusual chromosome abnormalities. 5) Identify factors controlling expression of H-Y antigen in persons with chromosome abnormalities. METHODS: 1) A comprehensive clinical and cytogenetic study on mentally retarded individuals with fragile X chromosomes or whose family histories indicate an X-linked form of mental retardation will be conducted. 2) Chromosomes of mentally retarded and control persons will be examined by high resolution banding of prometaphase cells. 3) Persons with chromosome abnormalities will have detailed clinical studies. 4) Relatives of persons with unusual abnormalities will be studied cytogenetically. 5) H-Y antigen tests will be done on the cells of persons with chromosome abnormalities.