Biomedical advances in developmental psychology: The case of fragile X syndrome

Fragile X syndrome, the most common inherited cause of mental retardation, is caused by an abnormal gene on the bottom end of the X chromosome. Discovered and sequenced in 1991, it is called the Fragile X Mental Retardation-1 (FMR-1) gene. Mutations in the FMR-1 gene include small expansions with a CGG (a specific sequence of the nucleotides) repetitive sequence that repeats from 50 to 200 times (the premutation) and the full mutation that involves a CGG repeat sequence that is greater than 200. In the full mutation, the FMR-1 gene is usually methylated, turning off the gene so that no protein is produced. Mutations within the FMR-1 gene can cause a spectrum of learning difficulties ranging from mild problems to severe mental retardation.