There have been a number of studies undertaken both in the “special needs” population and the general population aimed at determining the prevalence of FXS in males and females. The agreed upon prevalence of FXS in males is approximately 1 in 7,000 and in females 1 in 11,000.
The reason there are fewer females with FXS than males is that the gene for FXS is located on the X chromosome.
- Males (XY) — having only one X chromosome — will develop FXS because they have a mutation of their single (only) X chromosome.
- Females (XX) — having two X chromosomes — can have the unaffected X reduce the effects of the affected X, which typically leads to no or milder symptoms of FXS. (This is an important distinction, as many females with the full mutation do not consider themselves, nor are they considered by others, to “have” FXS.)