Fragile X syndrome is caused by a mutation in the Fragile X Mental Retardation 1, or FMR1 gene, according to MedicineNet. This is a gene that is found on the X chromosome and prevents the body from manufacturing normal levels of Fragile X Mental Retardation Protein, or FMRP.
The severity of the symptoms of Fragile X syndrome depend on how much of the protein the body is able to make, claims MedicineNet. If the protein is near normal levels, the symptoms are mild. However, if the protein is very low or absent, the symptoms of Fragile X are severe.
The FMR1 gene is prevented from producing FMRP because a sequence of amino acids in the gene repeat too many times, says MedicineNet. The sequence is cytosine-guanine-guanine, or CGG. In a person who does not have Fragile X syndrome, this sequence repeats between six and 40 times. However, in a person with Fragile X, this sequence can repeat hundreds of times. If the sequence repeats over 200 times, the gene is considered to be fully mutated. This mutation causes the gene to shut down in a process known as methylation. When this happens, the cell can't make the protein or makes too little of it.
The symptoms of fragile X can be complex and affects different individuals differently, says the National Fragile X Foundation. It affects their physical appearance, their intellect and their behavior.