Within the FMR1 gene is a specific region of CGG repeats. Individual with fewer than 40 repeats are not at risk to pass on fragile X syndrome to their offspring. Individuals with 55 to 200 repeats are said to carry the FMR1 premutation. Male individuals with more than 200 repeats have fragile X syndrome. There are a few reports in the literature documenting fragile X syndrome due to deletions of the FMR1 gene, but the vast majority of cases are the result of expansion of the CGG triplet repeat within the FMR1 gene.
Expansion occurs when the gene mutation is passed from the mother to the child.
If a father carries the gene premutation, he will pass the gene mutation on to his daughters in the premutation state. None of his sons will inherit the gene mutation. In individuals with more than 200 repeats, the gene usually becomes methylated resulting in the gene being turned off. When the FMR1 gene is turned off, FMRP is not produced. It is the lack of FMRP that causes the cognitive features and connective tissue findings of fragile X syndrome.