Dominant genetic and gender-linked diseases
Inside the nucleus of each cell are the genes and chromosomes that control how the body develops and functions. Genetic diseases happen when genes and chromosomes are abnormal.
Genetic disorders can be caused by a single defective gene, several faulty genes, or a fault in the number or shape of chromosomes.There may also be complicating environmental factors. A single defective gene that results in a genetic disorder can be either dominant or recessive, a mutation, or attached to the X chromosome. Abnormal chromosomes that result in genetic disorders are usually new mutations, but may be inherited (see Down's syndrome).
If either of you has a history of a genetic disease in your extended family, it's best to ask for genetic counselling before becoming pregnant. The number of tests available for genetic diseases is increasing every year, although they don't tell the potential severity of the condition.
Dominant genetic diseases
Fatal diseases due to dominant genes are rare because those affected normally die before they can pass on the genes. However, some, such as familial hypercholesterolaemia, can be managed.
This is the most common dominant genetic disease. Sufferers have such high levels of blood cholesterol that they risk heart attacks and other complications caused by narrowing of the arteries. This condition affects one in 500 people and can be detected by a blood test at birth.
Recessive genetic diseases
A defective recessive gene is usually masked by a normal dominant one. But if both parents carry a defective recessive gene, each of their children has a one in four chance of inheriting both recessive genes (and therefore one of several disorders) or neither, and a two in four chance of being a carrier. Thus there are always more carriers than sufferers.
Cystic fibrosis (CF)
This is the most common recessive gene disorder. One in 20 of the white population carries the CF gene, and one in 2,000 white babies born is affected by the disease. In non-whites, the incidence is about one in 90,000. This disease mainly affects the lungs and the digestive system. Mucus inside the lungs becomes thick and sticky, and builds up, causing chest infections. The mucus also blocks the ducts of various organs, particularly the pancreas, preventing the normal flow of digestive enzymes. If not treated promptly, CF results in malnutrition. Rapid and accurate carrier testing involving analysis of blood or mouth cells is possible. Over 60 per cent of sufferers survive into adulthood; some have been helped by heart and/or lung transplant surgery.
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