Fragile X syndrome (FXS) is caused by a mutation of the fragile x mental retardation 1 (FMR1) gene, which is located on the X chromosome. Children may inherit the abnormal gene from their mother, father or both parents. Parents can carry the abnormal gene that causes FXS but display no symptoms of the syndrome and, thus, unknowingly transmit it to their children. 

Boys are at greater risk of FXS than girls. Boys who receive the mutated gene from their mother usually develop a full-blown case of FXS – since boys have only one X chromosome, which they inherit from their mother. On the other hand, girls have two X chromosomes (they receive one from each parent). A second X chromosome in girls may compensate for, or reduce the effects of, an inherited X chromosome with an abnormal FMR1 gene.

The genetic transmission of FXS is different from other x-linked defects such as hemophilia. In FXS, both males and females can be carriers of the defective gene or be affected by the gene. In contrast, hemophilia is only transmitted from mother to son.

In addition, children who inherit the gene from their mother are at greater risk of FXS than children who inherit the gene from their father. The FMR1 gene mutation can worsen when it is transmitted from a woman to her child. Thus, a woman who is a carrier but has no symptoms of FXS can give birth to a child with full-blown FXS. On the other hand, the mutation does not worsen when it is transmitted from a man to his daughter. Thus, an asymptomatic (without symptoms) male who carries the faulty gene may pass it to his daughter, who may, in turn, become an asymptomatic carrier. Although she may not develop FXS, the daughter is at risk of giving birth to children with FXS.