A defect in a specific gene on the X chromosome, causing learning difficulties
Fragile X syndrome is caused by an abnormality of a specific gene on the X chromosome, one of the two sex chromosomes. This abnormality results in learning difficulties, and the condition is associated with a characteristic physical appearance. In the UK, about 1 in 4,000 boys is affected; the condition is far less common in girls since many women who carry the gene for the condition remain unaffected.
Girls with fragile X syndrome tend to have milder symptoms than affected boys because they have a second copy of the X chromosome that is normal and compensates for the abnormal gene on the other chromosome. However, any woman who carries this gene can pass it on to sons, who would develop more severe symptoms of the disorder.
The initial symptoms are learning disabilities, which vary from mild to severe and become more obvious as the child grows older (see Generalized learning disabilities). Affected children tend to be taller than normal. Other characteristic physical features of fragile X syndrome often do not appear until puberty and include:
Large head and a long, thin face with a prominent jaw and large ears.
In boys, large testes.
Affected children are also susceptible to seizures and behaviour problems (see Autism spectrum disorders).
If your child’s doctor suspects fragile X syndrome, he or she will arrange for a blood test to look for the gene abnormality on the X chromosome. Although the condition cannot be treated, affected children may benefit from speech therapy, specialized teaching, and help from a psychologist for behaviour problems. Some affected children need to take anticonvulsant drugs to prevent seizures. Parents of an affected child may wish to join a support group.
Genetic counselling is offered to couples who already have a child with fragile X syndrome to assess the risk of having another child with this condition. Antenatal testing can be performed in subsequent pregnancies (see Antenatal genetic tests).
The life expectancy for affected children is normal, but boys usually require lifelong care, either from their parents or in a residential home.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.