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Mental retardation, in particular the 'X-linked' type, has interested geneticists for many years. An increasing number of affected families have been to genetic counselling centres, and an effort is being made to find clinical and cytogenetic methods so a reliable diagnosis can be made. This would enable the detection of carriers and the opportunity to offer prenatal diagnosis. Many questions remain regarding X-linked mental retardation, its causes, diagnosis, and prevention. In this article we try to give an overview about the status of our present knowledge and the questions to be answered in the future. [ABSTRACT FROM AUTHOR]

The fra(X) frequencies in metaphases of lymphocytes from seven male patients with X-linked mental retardation and macroorchidism were scored after use of different culture techniques and different times of blood storage. No statistically significant differences were found between the fra(X) rates of lymphocytes grown either in folic acid deficient medium (TC 199) or in medium containing folic acid as well as a folic acid antagonist (methotrexate: MTX, aminopterine: AP). With respect to the effect of the time interval between blood sampling and culture set-up, a statistically significant decrease in the fra(X) frequency was observed in all culture types after 4 or 7 days of blood storage. [ABSTRACT FROM AUTHOR]

Investigation of lymphocyte cultures from three females heterozygous for fra(X)(q27) shows widely differing proportions of early and late replicating X chromosomes having the fragile site, and suggests that the replication status of the fragile X may be related to the mental capacity of the patient. The study has utilised a sequential staining technique to reduce ascertainment bias, and evidence is presented to suggest that the expression of the fragile site is independent of the differential incorporation of BUdR into the early and late replicating X chromosomes. [ABSTRACT FROM AUTHOR]