People with FXTAS have a change or mutation in the FMR1 gene found on the X chromosome.
In the gene, the information for making a protein has two parts: the introduction, and the instructions for making the protein itself. Researchers call the introduction the promoter because of how it helps to start the process of building the protein.
The promoter part of the FMR1 gene includes many repeats—repeated instances of a specific DNA sequence called the CGG sequence. A normal FMR1 gene has between 6 and 40 repeats in the promoter; the average is 30 repeats.
People with between 55 and 200 repeats have a premutation of the gene. The premutation causes the gene to not work properly, but it does not cause intellectual and developmental disability (IDD). FXTAS occurs in some people who have the premutation.
The premutation is also related to fragile X-associated primary ovarian insufficiency (FXPOI).
People with 200 or more repeats in the promoter part of the gene have a full mutation, meaning the gene might not work at all. People with a full mutation often have fragile X syndrome, the most common inherited form of IDD. Individuals with the full mutation are not at risk for FXTAS.
Anyone with the FMR1 gene mutation can pass it to their children. However, a person who inherits the gene may not develop FXTAS. Males will pass it down to all of their daughters and not their sons. Females have a 50/50 chance to pass it along to both their sons and daughters. In addition, parents can have children with a condition associated with fragile X even if the parents do not have that condition themselves.