Fragile-X mental retardation in a large five-generation family. A clinical and cytogenetic study.

A number of problems concerning both clinical and genetic or cytogenetic aspects of the fragile-X syndrome remain unsolved. In the present work, a large 5-generation fragile-X family has been clinically and cytogenetically investigated. The results of our study indicated that an unusually high proportion of affected males was present in the family examined; all fragile-X-positive males were profoundly retarded and showed the phenotype typically described for this syndrome; moreover, we observed a variability of penetrance within the pedigree and all fragile-X-positive females in the 4th and 5th generation had some degree of mental impairment. A multistep mutation model has been proposed in order to explain some of these findings.