Transmission of Fragile X

Because males normally have only one copy of the X chromosome, those males with significant tri nucleotide expansion at the FMR1 locus are symptomatic. They are intellectually disabled and may show various physical features of the fragile X syndrome.

Females have two X chromosomes and thus have double the chance of having a working FMR1 allele. Females carrying one X chromosome with an expanded FMR1 gene can have some signs and symptoms of the disorder or be normal.

Males with the fragile X cannot transmit it to any of their sons (since males contribute a Y chromosome, not an X, to their male offspring), but will transmit it to all of their daughters, as males contribute their X to all of their daughters.

Females carrying one copy of the fragile X can transmit it to their sons or daughters. Sons who receive the fragile X are at high risk of intellectual disability. Daughters who receive the fragile X may appear normal or they may be intellectually disabled, usually to a lesser degree than boys with the syndrome.