Pattern of inheritance

As the FMR1 gene is carried on the X chromosome, a woman with:

• full mutation has a 50% chance of passing the full mutation to her son or daughter
• premutation has a 50% chance of passing on the premutation; however, the gene may expand during transmission to her son or daughter, leading to the full mutation¹.

A man with fragile X syndrome premutation passes it on to all his daughters without it changing in size. His sons only receive his Y chromosome and cannot inherit the gene. A man with full FMR1 gene mutation would only pass on the premutation to offspring of either sex, but it is rare for males with fragile X syndrome to father children.

Extended family members across generations are likely to carry the gene and may be affected. However, in at least 30% of new cases of fragile X syndrome there is no known family history.

For genetic advice and support after a diagnosis, see here.