Fragile site X chromosomes and X‐linked mental retardation in severely retarded boys in a northern Swedish county. A prevalence study

Abstract

In an unselected series of 96 severely mentally retarded boys (IQ < 50) born 1959–70 in a northern Swedish county, six had a fragile site on the distal end of the X chromosome (FraXq 28). The prevalence of the fragile X syndrome in severely retarded boys was 6 %. Next to trisomy 21, this fragile X syndrome appears to be the most common single cause of severe mental retardation in boys.