Many genetic disorders affect boys more than girls, for a characteristic reason, as doctors at Great Ormond Street Hospital explain in our A-Z series on child health
What is an X-linked condition?
We have 46 chromosomes that we inherit from our parents. Among these are the X and Y sex chromosomes. Females have two X chromosomes, whereas males have one X and one Y chromosome.
X-linked conditions arise from genes on the X chromosome. Usually women carrying the faulty gene will not be affected because the other normal X chromosome will compensate for the faulty gene. However, because boys only carry one X chromosome, they will have a 50 per cent chance of inheriting this faulty gene and developing a genetic disorder.
X-linked conditions are relatively rare, but the following are some of the more common ones.
Duchenne Muscular Dystrophy
This is a fairly common X-linked severe neuro-muscular condition, affecting around one in every 3,000 to 4,000 males. A mutation in the genes responsible for muscle structure leads to progressive muscle weakness. The mother, if she is a carrier, will not display any signs of the disease.
Symptoms
- At the age of one to three most affected boys will experience mild delay with walking and tip-toeing and have problems running and climbing.
- Learning and/or behavioural difficulties are not uncommon.
- By the age of 11 the boy will be unable to walk due to a weakening of the muscles surrounding the ankles, knees and hips.
- By the time he reaches his teens or twenties his condition is life-threatening due to respiratory problems.
- More than 90 per cent of affected children who are confined to a wheelchair are at risk of developing scoliosis - curvature of the spine.
Diagnosis and treatment
While a great deal can be done to control the effects of DMD there is no cure to halt the loss of muscle cells. Pre-natal and carrier detection may be available in specialist clinics.
Haemaophilia
Haemophilias are a group of inherited blood disorders caused by defects in the blood clotting system. Haemo- philia A and B are X-linked conditions and almost always affect men.
Symptoms
- These can range from bruising to continuous bleeding.
- Bleeding can be caused spontaneously or from an injury.
- Repetitive bleeding can result in arthritis and long-term joint damage.
Diagnosis and treatment
Blood tests are available to identify carriers of the disease. Pre-natal diagnosis can also be easily performed from ten weeks of pregnancy. A number of hospitals and haemophilia care centres across the UK offer treatment. Children who are severely affected can receive regular injections to prevent bleeding.
Fragile X syndrome
This is the biggest single inherited cause of learning disability. It affects one in 2,000 males and one in 4,000 females. It is caused by one of the X chromosomes having a faulty gene. Women are often silent carriers of the condition, with a 50 per cent chance of passing on the faulty gene to their children.
Symptoms
- Learning disability, which can vary from mild to severe.
- Emotional and behavioural problems, hyperactivity and attention deficit disorder.
- Physical characteristics including a long face with big ears, a large jaw and flat feet.
- Intellectual development appears to decline with age.
Diagnosis and treatment
There is no cure for Fragile X. Several support networks provide educational and emotional help to children and their families. Diagnosing Fragile X is easily done through a DNA test. Pre-natal testing is also possible.
Genetic testing
Genetic testing is available in cases where the parents are aware that they are carriers, already have a child with a genetic condition or have a history of the disease in the family.
If a child in your care has a genetic disorder and the parents want to get a diagnosis on whether this can happen again, they should be advised to visit their GP, who will refer them to a genetics clinic.
Researched and written by Marcella McEvoy
Further help
Contact a Family, a UK charity for families and carers of disabled children, has an excellent website, www.cafamily.org.uk , which provides comprehensive information on rare conditions
