fragile X syndrome

The most common inherited cause of mental retardation. It is inherited as an X-linked recessive condition and is so named because sufferers have a fragile site on one of their X chromosomes. Women carry two X chromosomes and are therefore more commonly affected than men, but less seriously.

It affects up to 1 in 1,000 boys and 1 in 2,500 girls. Virtually all male sufferers are mentally retarded, and while some of the females are unaffected most have significant learning difficulties.

The fragile site is caused by a mutation that causes unstable forms of the gene FMR-1 to expand by repeating three specific nucleotides. Healthy carriers may have about 60 repeats; those severely affected may have thousands. The gene was located in 1991.