Fragile X syndrome Dr Trisha Macnair Fragile X syndrome (or Martin-Bell syndrome) is the most common inherited cause of learning disability.

In this article

What causes it? What are the symptoms?

Who's affected? How is it diagnosed?

What's the treatment? Advice and support

In fragile X, one of the genes on the X chromosome is faulty. A constriction can be detected near the tip of the X chromosome, which makes it fragile and susceptible to breakage.

Boys are usually more severely affected by the syndrome, as they have only one X chromosome. Girls have a second X chromosome, which can be used instead of the faulty one, and may have only mild disabilities. They may also be silent carriers of the condition.

The main problem in fragile X is mental impairment. This may range from a normal IQ to severe learning disabilities. Other symptoms include hyperactivity, attention deficit disorder, emotional and behavioural problems, anxiety and mood swings.

There may also be characteristic facial features, such as a long face and large ears. Other physical features include flat feet and hyperextensible joints.

Fragile X affects about one in 4,000 males and one in 8,000 females of all races and ethnic groups

Fragile X affects about one in 4,000 males and one in 8,000 females of all races and ethnic groups.

About one in 259 women and one in 800 men of all races carry fragile X. It shows an X-linked recessive pattern of inheritance, but with some oddities that haven't been fully explained.

For example, about one in 800 males is a carrier and not affected (even though they have no normal copy of their X chromosome). However, males who have the full mutation are almost always affected. Most recent studies indicate that 10 to 15 per cent have mild learning disability/borderline normal IQ, 75 to 80 per cent are moderate learning disabled and 10 per cent have severe learning disability.

One woman in 259 is a carrier of fragile X. Premutation carriers (those who don't have the full mutation) don't have learning disabilities. In women with the full mutation, one third are below normal IQ, one third have a borderline IQ and the remaining third have a normal IQ.

Many people who have the gene don't have a fragile chromosome. These complexities have important implications for the risk of passing the disorder on through a family.

Fragile X can easily be diagnosed using a DNA test. Antenatal testing is possible using chorionic villus sampling or amniocentesis.

As yet there are no specific treatments or a cure for fragile X, but emotional and educational support for children and their families are vital to help them reach their best potential.

The Fragile X Society

Tel: 01371 875100
Website: www.fragilex.org.uk

This article was last medically reviewed by Dr Rob Hicks in November 2006.