Home | What is Fragile X syndrome? | How is Fragile X inherited?
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How is Fragile X inherited?

From our parents, we get forty-six chromosomes that contain our DNA and determine how tall we are, what colour eyes we have, etc. Grouped into 23 pairs, we get one set from our mother and the other from our father.

One of these pairs of chromosomes determines if a person is male or female:
  • Males (XY) get the X chromosome from the mother and the Y chromosome from the father.
  • Females (XX) inherit one X chromosome from each parent.
The Fragile X premutation can be passed silently down through generations in a family before a child is affected by the syndrome.

Carrier men pass the premutation to all their daughters (who receive the “Fragile” X chromosome) but none of their sons (who receive the Y chromosome).

Each child of a carrier woman has a 50% chance of inheriting the gene defect because she could pass on either the normal X or the “Fragile” X chromosome.

The “Fragile” X chromosome could stay the same length (premutation) or grow (full mutation). So a pregnancy could result in:
  • a boy or a girl who is clear of Fragile X
  • a boy or a girl who has the premutation and is a carrier of Fragile X
  • a boy who has the full mutation and is affected by Fragile X
  • a girl who has the full mutation and is possibly affected by Fragile X

The silent carriers of Fragile X syndrome

No family history of Fragile X? You may be surprised. Fragile X can be passed down through many generations in a family before a child is affected by the syndrome.