Heritable fragile sites on human chromosomes. VIII. Preliminary population cytogenetic data on the folic-acid-sensitive fragile sites.

The incidence of the autosomal folic-acid-sensitive fragile sites in 524 institutionalized retardates (.0095) was found to be significantly higher than in 1,019 unselected neonates (.00098), suggesting that heterozygosity for these fragile sites may not be as harmless as previously thought. When one of the parents of an index case was found to carry the fragile site, that parent was always the mother. The fragile site at Xq27 was not found among the neonates studied, but was present in 1.6% of the institutionalized retarded males examined; if this fragile site occurs in normal males, then it does so rarely. Further cytogenetic studies of fragile sites are required on both normal and abnormal populations.