Understanding Genetic Causes of Intellectual Disability: A Look at Key Syndromes

Down syndrome is caused by the overexpression of genes on the extra copy of chromosome 21, but many mental retardation syndromes are due to mutation in a single gene.

For instance, Fragile X syndrome, which is the most common inherited form of mental retardation, is caused by a expansion of a CGG trinucleotide repeat in the FMR1 gene on the X chromosome.

Rett syndrome is an X-linked dominant mental retardation disorder that is a common cause of severe mental retardation and autistic features in girls, and the disease is caused by mutations in the MECP2 gene.