Fragile-X mental retardation syndrome transmitted through intellectually normal males: implications for genetic counseling.

The fragile-X mental retardation syndrome is the second most common identifiable cause of mental retardation in man. This condition violates many of the expectations for X-linked disorders, including the transmission of the syndrome through men who carry the gene but, for unknown reasons, do not express it. Two new cases of male transmission are presented along with four other cases heretofore unidentified in the literature, bringing the total number of confirmed or probable cases of transmission through normal men to 32. The various unorthodox characteristics of the syndrome are reviewed in light of their influence on genetic counseling. Recommendations for counseling families with fragile-X include evaluating all sons of carrier women psychometrically and cytogenetically, abandoning termination of pregnancies with male fetuses as a means of preventing the fragile-X syndrome, assuming that all mothers of sporadic cases are carriers, and karyotyping at-risk female members at an early age.