Fragile (X) X-linked mental retardation. II. Frequency and replication pattern of fragile (X)(q28) in heterozygotes.
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.
D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D004261:DNA Replication; D005260:Female; D005600:Fragile X Syndrome / Q000235:genetics* / Q000523:psychology; D005786:Gene Expression Regulation; D006579:Heterozygote; D006801:Humans; D008297:Male; D008875:Middle Aged; D012729:Sex Chromosome Aberrations / Q000235:genetics*; D013997:Time Factors; D014888:Wechsler Scales; D014960:X Chromosome