Home  /  Risks of having a child with FXS

Risks of having a child with FXS


Your risk of having a child with fragile X syndrome is related to the length of the gene associated with fragile X syndrome: short, intermediate, medium or long.

The gene can get longer when passed from mother to child but not father to child.

Gene length Effect Chance of having a child with fragile X syndrome
Short Normal Extremely low chance. All children will inherit the short gene.
Intermediate Grey zone Extremely low chance.
Women have two X chromosomes and pass only one to each child. There is a 50% chance (1 in 2) for each child inheriting the altered gene.
If the child does inherit the altered gene there are two possibilities.
  • The gene stays the same size and the child has an intermediate length gene.
  • The gene increases in size to the medium length gene and the child is a carrier of fragile X syndrome.
Medium Carrier
Also called premutation
Yes.
Women have two X chromosomes and pass only one to each child. There is a 50% chance (1 in 2) for each child inheriting the altered gene.
If the child does inherit the altered gene there are two possibilities.
  • The gene stays the same size and the child is a carrier of fragile X syndrome.
  • The gene increases in size to the long length gene and the child is affected with fragile X syndrome.
Long Affected
Also called full mutation
Yes.
Women have two X chromosomes and pass only one to each child. There is a 50% chance (1 in 2) for each child inheriting the long length gene and being affected by fragile X syndrome.

What about male carriers of fragile X syndrome?

The length of the gene can increase in size when passed from mother to child but not when it is passed from father to child.

It is very unlikely that a male carrier of fragile X syndrome would have a child with fragile X syndrome.
Males with the intermediate or medium length gene (carrier) will pass it on to all of their daughters and none of their sons (they pass their Y chromosome on to their sons). Thus their daughters will be carriers of fragile X syndrome.