Fragile X trait in a large kindred: transmission also through normal males.

In the pedigree reported here two apparently normal males may have transmitted the fragile X chromosome. Eighteen family members were examined cytogenetically. The fragile X was detected in a high percentage of cells from nine mentally retarded members of this family (six males and three females) and in one female obligate carrier. Four other obligate carriers showed no or only a few cells with the fragile X.