Reference lists On-line information Fragile X syndrome affects about 1 in 4000 males and 1 in 8000 females. There is no cure but there is some evidence that treatment of the associated behavioural and educational problems can be beneficial. Genetics Fragile X syndrome is caused by mutation of the FMR-1 gene on the X chromosome . This sequence occurs in a part of the gene that is transcribed but is not translated into protein. The full mutation prevents transcription of the FMR-1 gene, so that none of its protein product is made.
Males have only one X chromosome, so if they carry a FM they are always affected.
The population frequency of the PM is about 1 in 250. For reasons that are as yet not understood, the number of repeats in a PM is potentially unstable and can increase into the FM range in a child that inherits the affected chromosome from its mother. In contrast to the potential instability of a PM transmitted from the mother, a PM transmitted from the father does not expand to a FM in his daughters. Implications of current knowledge of the genetics of fragile X syndrome Genetic testing for fragile X mutations DNA-based testing for a FM or PM is very accurate. An additional question concerns the justification of diagnostic testing in individuals who are suspected as having the syndrome.