Screening of mentally retarded males for macro-orchidism and the fragile X chromosome.

Four hundred forty-four male residents of a state mental retardation institution were screened for macro-orchidism. Twenty-six white males (8.3%) and two black males (1.5%) had marked macro-orchidism (greater than 34 ml). Seven of 17 whites tested for the fragile X were positive; the one black tested was negative. Thus, a minimum of 7/26 or 27% (whites) are fragile X positive indicating potential population variability, also evident from previous reports. Concurrent testing of institutionalized brother pairs indicated over half of the fragile X-positive males had a strong family history consistent with X-linked mental retardation.