In 1991, scientists discovered the gene (called FMR1) that can cause
fragile X. The FMR1 gene is located on the long arm of the X chromosome.
Within this gene lies a region of DNA which varies in length (the # of repetitions
of the CGG trinucleotide) from one person to another. Ordinarily, this stretch of
DNA falls within a range of length that would be considered "normal"
(5-50 repetitions).

In some people, however, this stretch of DNA is somewhat longer and this
gene change is called a 'premutation' (50-200 repetitions). These individuals
generally do not show symptoms of fragile X but are considered carriers.

In carriers, this stretch of DNA is prone to further expansion when it is passed from a woman to her children. When the stretch of DNA expands beyond a certain length, the gene is switched off and does not produce the protein (FMRP) that it normally makes. This gene change is called a 'full mutation' (over 200 repetitions). These individuals have fragile X syndrome and can exhibit a range of characteristics.

A male who inherits a full mutation exhibits characteristics of fragile X syndrome because his only X chromosome contains the mutated gene. A female may not be as severely affected as a male because each cell of her body needs to use only one of its two X chromosomes and randomly inactivates the other.

Testing for Fragile X Syndrome:

The fragile X DNA test has revolutionized fragile X syndrome diagnosis and accompanying genetic counseling. It has been available since 1991 and provides definitive diagnosis of fragile X syndrome and extremely accurate carrier detection. Reliable for people of any age, it can also be performed prenatally. It has superseded the fragile X cytogenetic test due to its greater reliability and accuracy in diagnosis and its ability to identify unaffected carriers.

Many asymptomatic carriers of fragile X syndrome are unaware they are carriers because there may not be a family history of fragile X syndrome, or because a relative with fragile X syndrome may not have been diagnosed. The carrier rate in females is quite high, at approximately 1/129. It is recommended that fragile X carrier testing be offered to all women of reproductive age who have a relative with mental retardation of unknown cause.

For detailed information on testing please click on the following link: