home fragile what? characteristics resources donate awareness events photos contact


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 It has superseded the fragile X cytogenetic test due to its greater reliability and accuracy in diagnosis and its ability to identify unaffected carriers.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 1/300. 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: http://www.fragilex.org/html/testing.htm