Fragile X syndrome is the most common form of inherited mental retardation.
Fragile X happens when there is a change, or mutation, in a single gene called the Fragile X Mental Retardation 1 (FMR1) gene. This gene normally makes a protein the body needs for the brain to develop. But when there is a change in this gene, the body makes only a little bit or none of the protein, which can cause the symptoms of Fragile X.
Fragile X is inherited, which means it is passed down from parents to children. Parents can have children with Fragile X even if the parents do not have Fragile X themselves. The changes in the gene can become more serious when passed from parent to child.
Some people may only have a small change in their FMR1 gene (called a premutation) and may not show any signs of Fragile X. Other people may have bigger changes in the gene, called a full mutation, that cause the symptoms of Fragile X Syndrome.
For more information about genes and chromosomes, see Cells 101.