| Abstract: | The frequency of cytologic expression and the replication pattern of the fragile (X) [fra(X)] were investigated in 28 fra(X) heterozygotes, of which 25 agreed to psychological assessment. One-third of the heterozygotes in this study are mentally retarded. The intellectually impaired carriers had a higher frequency of fra(X) and a higher proportion of early-replicating fra(X) than the normally intelligent carriers. The early-replicating fra(X) accounted for 39% of the variability in IQ and the late-replicating fra(X) for 12%. Age had a minimal inverse effect on fra(X) expression and replication pattern. Thus, it appears that mental retardation in females heterozygous for the fra(X) may largely be a function of the proportion of cells with an early-replicating, active X chromosome possessing the fragile site. |
