How Does Fragile X Run in Families?
The FMR1 gene is on the X chromosome. Like all chromosomes, the X chromosome is passed from parent to child. Females have two X chromosomes, and males have one X chromosome and one Y chromosome. To see how X and Y chromosomes pass from parents to children, see About X-linked conditions. People with a normal number of repeats In addition to being X-linked, the inheritance pattern of FXS in families is affected by the number of CGG repeats in the gene. Most people have about 6 to 45 repeats in their FMR1 genes. Repeats of this size are stable in families. That is, the number of repeats does not change when passed from parent to child. People with an intermediate number of repeats People who have an intermediate repeat size (about 45 to 55) are not at risk for having children with FXS. However, when the gene is passed on to their children, the number of repeats can grow to a premutation size. Children who have a premutation are then at risk for having children with FXS. Therefore, a perso
