A carrier has an altered form of a gene that can lead to having a child or grandchild with a genetic disorder. We are all carriers of gene mutations, many of which are “silent.” This means we can pass the gene on but suffer no ill effects from it ourselves. It is only through genetic testing that we know which ones we carry. Some genes are on the non-sex chromosomes (numbered 1-22), which are the same in males and females, and some are on the sex chromosomes, which are the X and Y chromosomes.
Traditionally, a carrier of a genetic mutation is defined as a person who inherits an altered form of a gene but shows no effects of that mutation. However, in Fragile X this definition does not exactly fit, as carriers of a premutation are at risk to develop Fragile X-associated Disorders including Fragile X Tremor Ataxia Syndrome (FXTAS) and Fragile X associated primary ovarian insufficiency (FXPOI).
Males and females can both be carriers of Fragile X. Carrier status, as opposed to someone with the full mutation, is most easily defined by the number of repeats of the Fragile X (or FMR-1) gene. A normal range of this gene for an unaffected person is between 1-59 CGG repeats. A carrier, or person with the Fragile X premutation has between 59-200 CGG repeats.
This means that although the repeat has increased in size, the gene is still functioning and producing a protein vital for normal brain functioning. However, in female carriers, the repeat size can be unstable and increase in size as it is passed on to future generations.
Only female carriers can have full mutation children. Full mutation occurs when the repeats of the Fragile X gene increase to over 200 (anywhere into the 1000’s). This increase or lengthening of the gene prohibits the production of a protein vital for normal brain function.
Research has shown that although carriers are often largely unaffected by this premutation, they can have some symptoms of Fragile X and also may share/or suffer from a set of common traits.
The Fragile X Male Carriers booklet from the UK Fragile X Society reports the findings of a study of men who carry the Fragile X premutation.