X-linked mental retardation: transmission of the trait by an apparently unaffected male.

A pedigree is presented in which an apparently unaffected man transmitted the gene for X-linked mental retardation to at least four of his 12 daughters. None of his 12 sons was mentally retarded. These findings may be explained by a somatic mutation and germinal mosaicism in the father or by a half chromatid mutation in maternal gametes.