Many inborn diseases show this effect. The recessive abnormality phenylketonuria (or PKU) affects about forty British children a year. V.Ach has an inherited defect in a particular enzyme which means that they cannot process an amino acid, phenylalanine, found in most foods. As a result they build up large amounts of a harmful by-product. Untreated, such children have low intelligence and die young. The fate of those with PKU is, it seems, sealed by their genes.

Rut most PKU children born today lead more or less ordinary lives. A change in the environment saves them. If they are diagnosed early (and all babies are tested at birth), they can be given food which lacks all but a tiny amount of phenylalanine. They then develop as healthy infants. Their nature has been determined by careful nurturing and the question of whether DNA or diet is more important to their health has no answer.

Hundreds of genes show the same interaction. A whole new science turns on individual differences in the response to drugs. The genes involved were unknown until humans began to manipulate their chemical milieu. A few people carry an inherited variant which makes them fatally sensitive to a muscle-relaxant used before surgery and everyone is now tested to see whether they are at risk before the drug is given. One of the stranger injunctions of Pythagoras was a caution to his followers not to eat broad beans. He died because, pursued by a mob enraged by his philosophical views, he refused to escape across a beanfield. Pythagoras lived in the Italian city of Croton. Many of its modem inhabitants feel unwell if they eat partly cooked beans. One of the side-effects of the thalassaemia gene (which is common there) is to remove the ability to break down a chemical found in broad beans {and another one used as an anti-malarial drug). When gene and bean (or drug) are brought together, the results can be unpleasant or, in the case of the drug, worse.

All this means that the boundaries between inherited disease and what is governed by the extern.il world have become blurred. That alters the way we think abiuii medicine. Individual treatments may soon be tailored to a patient's biological heritage. Two disorders, anencephaly and spina bifida, cause a failure of development of the spinal cord; and each runs in families. Part of the problem, though, has to do with poor diet. Their incidence shot up in Holland after the famine of 1945 and both are frequent in Ireland and in Scotland (places known for an unhealthy diet). Mothers who have had an affected child now take vitamin supplements in later pregnancies. This reduces the chance of their genes damaging their children.

A change in the environment can also cause genetic disease. Hay fever was not recognised as a distinct illness until 1819, when it (and its relatives asthma and eczema) were seen as afflictions of the rich. Now, about half the people of the western world are, or claim to be, allergic to one substance or another. In Britain one child in four has asthma. The lung becomes inflamed and its muscles sensitive to the slightest irritation. The unfortunate patient wheezes and coughs, and may suffer permanent damage — and, sometimes, even sudden death. The illness involves an over-reaction by the immune system to an external stimulus. House-dust mites are one culprit, cats another, pollen a third. They were around before 1819, but, for some reason, caused few problems.

Part of the reason lies in the modern world, with its obsession with cleanliness. This may have abolished many infectious diseases, but allows others, once rare, to reveal our inborn weaknesses. Asthma is a disease of the middle class; more common in those well fed as children, in infants dosed with antibiotics, and in Western rather than Eastern Europe. The children of farmers and of those with dogs have less chance of the illness than do vegetarians in a pet-free home. It is an affliction of Thrushcross Grange rather than of Wuthering Heights.

Emily Bronte knew the answer. Cathy, when she returns clean and demure after her convalescence at Thrushcross Grange is faced by Heathcliff's: 'I shall be as dirty as I please; and I like to be dirty, and I will be dirty!' Filth is the key. Infants born into clean households are deprived of an essential learning experience. Not their brain but their immune system lacks stimulation. Middle-class homes lack the grime with which humankind evolved. The immune system, like the brain itself, must be trained to deal with the challenges that it will face later in life. Each needs stimulation; but the immune system demands tapeworms rather than Mozart.

Whatever the importance of dirt, asthma and its relatives have an inherited component. Identical twins are more likely each to suffer than are non-identicals, and those who bear certain variants in genes that code for elements of the immune system are also liable to become ill. Tristan da Cunha, that distant and inbred island in which many people share the same genes, has an epidemic of asthma, with almost half the population affected.

Allergy is a classic of the interaction of gene and environment. Long ago, the genes that today cause problems may have been useful as those with an active immune system were good at resisting infection. After soap, in an unnaturally clean  household,  an over-active  immune  system became a nuisance as it disposes to allergy. The environment has changed, but the genes remain the same. Today's DNA has quite different effects on health than it once did, and a change in the interaction of nature with nurture leads to an outbreak of illness.

The term 'cancer' covers a multitude of conditions. All are due to a failure to control cell division. Hundreds of genes control the growth of cells anil, when they mutate, the process may go out of control. As in ****** all kinds of mistakes can happen. A single DNA I use may change or whole sections of the message be lost. Sointlitnes the error involves genes moving from one chromosome to another, or from the effects of viruses. Often, several different genetic accidents are needed to promote the development of a tumour. The general picture is not much different from that of mutation in sperm or egg.

Cancer is a Siamese cat of an illness, and the chances of contracting it depend both on the genes and the circumstances with which they are faced. Cell division needs brakes and accelerators. The first, tumour suppressor genes as they are called, control a set of proto-oncogenes that encourage cells to grow. If either party goes wrong, then division speeds up. The cell, though, has a set of speed cameras that control rogue genes. It must pass through a number of checkpoints on the road to division, and if anything is suspicious the cell dies (which is, after all, the natural fate of most cells}. Many of the causes of cancer increase the amount of a specific protein — P53, as it is called — that is sensitive to any sign of DNA damage in the cells under attack. Most then commit suicide rather than causing trouble. Indeed, many cancer treatments (themselves often agents of the illness, like radiation and certain chemicals) themselves wake up the P53 genes and persuade the cells to do the decent thing. Damage to the checkpoint itself (either inherited, or caused by the external agent) means real trouble: by that time the rogue line of cells is through the last safety barrier and may be impossible to contain.

Some cancers are more common among those exposed to a particular hazard. Many chimney-sweeps died of a skin cancer, which appeared first on the scrotum. The English physician Percival Pott suggested that soot was to blame. He was right. Soot, oil and tar contain many carcinogenic chemicals. Radiation, too, can be dangerous. As many as two thousand cases of lung cancer per year in Britain — a twentieth of the total — arise from exposure to radon. There were once thought to be clusters of childhood leukaemia cases around nuclear power-stations but these have now been dismissed on statistical grounds. For most Britons, exposure to radiation is so low that it cannot be an important general cause of cancer.