Fragile X syndrome (FXS) is an inherited disorder affecting intellectual, behavioral, and social development. It occurs in both males and females who have a full mutation of the FMR1 gene.
The FMR1 gene is on the X chromosome. Males have one X and one Y chromosome; females have two X chromosomes. In females with a full mutation, their other, normal X often compensates for the FMR1 mutation. That frequently results in milder symptoms of FXS. In males the Y chromosome cannot compensate for the effects of the Fragile X mutation.