Normal male carriers in the fra(X) form of X‐linked mental retardation (Martin—Bell syndrome)

Abstract

Evidence for the transmission of X‐linked mental retardation through normal male carriers is reviewed in 6 kindreds. In these pedigrees we identified 15 unaffected males who likely had passed the gene on through their daughters. Fifty‐one mentally retarded grandsons or great grandsons descended from these males had 50 daughters with only 2 of them being of low intelligence. Two of the male carriers were recently identified through fra(X)‐ positive results in their mentally normal daughters. Among the sibs of these males, mentally retarded brothers were found in 3 families. This was unexpected since earlier observations suggested that the risk for mental retardation among sibs of non‐manifesting carriers is exceedingly low.