Out of dnaL: mapping the medical maze.

An estimated 250 million people, or 4.5 per cent of the world population, carry a potentially pathological haemoglobinopathy gene.

Every year, 300,000 infants are born with major haemoglobin disorders, the most common being the thalassaemia and sickle-cell disorder.

Prevalence varies from under 0.1 births per 1,000 in some parts of the worm to more than 20 per 1,000 in parts of Africa.

Worldwide, about 5 per cent of children are born with a congenital or hereditary disorder and almost 40 per cent of adults are genetically predisposed to common diseases during their lifetime.

In developed countries, even under the best of conditions, congenital and genetic disorders account for a quarter of deaths under the age of one and 23 per cent between one and four.

Until recently, coronary heart disease was thought to be due to lifestyle alone. Factors such as smoking, diet and exercise play a major part, but a family history will often uncover genetic susceptibility. For example, a heart attack before the age of 55 years in a relative presents a significant risk.

Evidence started to accumulate about 25 years ago that risk factors such as high cholesterol levels and high blood pressure are genetically influenced. Recent research has uncovered "candidate" genes for other risk factors.

An estimated 3 million infants annually are born with major congenital malformations and most die by three. But, if all goes well, all human genes would have been mapped and identified by 2020.

The last 20 years have seen an explosion of knowledge about the role genes play in health, from the moment of conception to the day people die. DNA determines not only the congenital malformations that kill millions of children, but also predisposes people to mental illness and major noncommunicable diseases (NCDs) such as cancer, cardiovascular disease, hypertension, asthma, diabetes and rheumatoid arthritis. Single-gene defects, genetic predisposition and congenital malformations together form a large group of genetically determined conditions. Single-gene defects, such as cystic fibrosis, haemoglobin disorders and haemophilia, are an important cause of fatal or chronically disabling diseases, especially in childhood. More than 9,000 single-gene conditions have now been identified. They occur worldwide, although some are more common in certain ethnic groups or geographic areas. Their incidence may be affected by differing demographic and cultural factors, such as maternal age or...