The fragile X‐chromosome: an evaluation of the results in a routine cytogenetic laboratory in the period 1981–1986

We report on the cytogenetic studies, performed in a routine cytogenetic laboratory between 1981 and 1986, on 428 subjects: 291 probands with non-specific mental retardation, 101 first-degree relatives of fra(X) positive patients and 36 non-retarded patients, referred for other reasons. As a rule 50 cells, cultured in folate-deficient medium were analysed. The results are compared with data collected from the literature and with the data expected from prior genetic risks and fra(X) penetrance. If no more than 50 cells were analysed, the fragile site was found in a lower than expected proportion (69.2%) of the retarded first-degree male relatives. The probability of misclassification can be calculated for various levels of fra(X) expression. We conclude that in a substantial number of unselected patients with mental retardation the diagnosis fra(X) syndrome will only be confirmed cytogenetically if at least 100 cells are analysed. Five percent of the male and 10% of the female index patients showed a fragile X-like abnormality, probably not associated with the Martin-Bell syndrome.