title: Activity of the fragile X in heterozygous carriers.
abstract: Chromosome analyses with conventional stain, Q- and G-banding, and R-banding with 5-bromodeoxyuridine (BrdU) incorporation were performed on the lymphocytes of two sisters who are heterozygous for the fragile X chromosome and clinically diagnosed as slow learners. Two heterozygous relatives with normal intelligence were used as controls. The frequencies of the active fragile X for the "slow" females were 100/129 (77.5%) and 85/120 (70.8%) compared with 40/78 (51.3%) and 10/32 (31.3%) for controls, the difference being highly significant. These observations are consistent with the Lyon hypothesis: activity of the abnormal X could account for the reduction in mental ability of some heterozygous females. Similar to retarded males with the fragile X chromosome, our slow learners had verbal scores that were lower than performance scores.
mesh_terms: D000328:Adult; D002871:Chromosome Banding; D002873:Chromosome Fragility; D004303:Dosage Compensation, Genetic; D005260:Female; D006579:Heterozygote; D006801:Humans; D008607:Intellectual Disability / Q000235:genetics*; D008214:Lymphocytes / Q000648:ultrastructure; D008297:Male; D008875:Middle Aged; D010375:Pedigree; D012730:Sex Chromosomes; D013194:Staining and Labeling; D014888:Wechsler Scales; D014960:X Chromosome