If the rate of mutation to haemophilia is taken as the norm, there must be about one new DNA change in a functional gene per five generations. This means ten million changes in functional genes per generation in Britain. The actual incidence may be even higher. Hormonal changes in women who are attempting to become pregnant show that eight out of ten fertilised eggs are lost. Many may carry new lethal mutations. Often, they involve the loss of all or part of a chromosome, and the incidence of such errors in still-born children is ten times that among those born alive.

Each gene has its own mutation rate. The frequency varies more than a thousand times from gene to gene. Larger genes with more interspersed pieces of DNA go wrong more often than smaller ones, and certain combinations of bases change more readily than others. The short segments of repeated DNA outside the functional genes (such as those involved in the 'genetic fingerprint') have a high rate of error. As many as one person in ten may pass on a change. The rate of mutation itself has evolved, too, and is controlled by enzymes which can repair injured DNA. When these are themselves damaged it shoots up.

Many things increase the mutation rate. Radiation, for example, can have a powerful effect. Plenty of mutations do not arise from the natural instability of the genes, but from damage inflicted from outside (as many of the early workers on radium, many of whom died of cancer, soon found out). Up to two thirds of the sperm cells of cancer patients who have been given large X-ray doses carry chromosomal changes. Evidence from other animals makes lower doses of radiation a real cause for concern, given the link between agents that cause mutations in sperm and egg and those that cause cancer. The acceptable dose for humans is set in part by research on mice (which seem to be more susceptible than we ourselves are) and there have been calls to have the limits increased; but nobody denies that radiation damages our genes. Sunlight, too, is harmful to cells and even an increase in temperature can increase the rate of error.

The biggest avoidable source of radiation in Britain is radon gas, which leaks from granite. People who live in granite houses in Cornwall may be exposed to more excess radiation than are those who work in nuclear power stations (although their equivalents in the granite city of Aberdeen may in part reverse the effect as they wear the kilt, with its cooling influence). In the United States, houses built with radioactive sands in their foundations have been demolished as their occupants faced twenty times the average dose. In the UK, those at risk are advised to install fans to stop the build-up of gas. Other sources of radiation include the cosmic rays experienced during air travel and medical X-rays, but for most people these involve very small doses.

Chemicals are much more important agents of genetic damage. The number of chromosome errors in nuclear power-station workers is ************ that of the general public, but the number in those employed in coal-fired stations is even higher because ****** noxious byproducts of burning coal. Bacteria are used to test a huge number of likely, and some unlikely, substances. Some, such as those once used in hair dyes, had a powerful effect and have been banned. Others, those in black pepper, in Earl Grey tea and in some pesticides, also cause mutations. Some of the most potent are quite natural. Plants produce many toxic chemicals for defence against insects and even lettuce, the epitome of a healthy diet, contains substances that cause mutations in mice. Almost half of all cancers may be influenced by the food we eat; and the vast majority of the pesticides — perhaps more than 99.9 % — in the Western diet are perfectly natural. Cynics argue that organic foods are more dangerous than food which has been sprayed because of the noxious chemicals found in the moulds which grow on them. Fresh fruits and vegetables reduce the rate, and some plants, such as broccoli and tomatoes, are filled with anti-mutagens that may help protect those who eat them.

Mutations are the raw material of evolution. Life progresses; it does not decay, but every individual is mortal. As we grow old our machinery corrodes until at last it breaks down.

Parr of this erosion comes from genetic changes within the body and part from the delayed effects of genes advantageous when young but harmful when old. To build an adult from a fertilised egg involves making hundreds of millions of cells, each with its own copy of the original message. The copying process is imperfect, and there are plenty of chances for mistakes. Even in adulthood most cells continue to divide. Red blood cells, for example, are renewed every four months or so. Kvcry minute everyone makes thousands of miles of DNA. As a result, huge numbers of mutations build up in body cells. Each individual is an evolving system whose identity changes from day to day.

Some of these changes can lead to disaster. Many cancers result from genetic accidents. Indeed, some cancers look more and more like genetic diseases. They represent a decay of the genetic message and a loss of control by DNA of the cells in which it lives. Age is a reflection of the same process. As our bodies are in a constant fever of replication, the older we are the more divisions there have been and the more chance for error. The cells of a new-born baby are separated by just a few hundred divisions from the egg; but mine, as a fifty-something-year-old are distanced from it by thousands. My genes have had more chances to mutate than have those of a baby. What is worse, they are less effective at repairing the damage. The cells of old people even contain altered genes which make inappropriate proteins. Thus, many aged Europeans have small amounts of sickle-cell haemoglobin in their blood. This gene is normally found in Africans but has, in their case, appeared as a new mutation within their elderly bodies.

Ageing accelerates with age. The lowest risk of death is at about the age of twelve — just before puberty. After that, the rate doubles every eight years or so, giving a seventy-six-year old about a two hundred and fifty times greater chance of dying than a teenager. The power of accelerated decay is impressive. If the death rate stayed at that of a twelve-year-old, most people would live to a thousand and there would be a small but noticeable proportion of people around who were born in the last Ice Age. Unfortunately, our obsolescence is such that even centenarians are rare. All this helps to explain why cancer is a disease of the old; so much so that even if the disease were eliminated altogether lite expectancy would go up by only about four years. The biological identity crisis which we define as old age and which is solved by deaih happens when the genetic message becomes so degenerate th.u its instructions no longer make sense. The rate ol ageing is programmed. Mouse cells in culture stop dividing after about four years, while human cells can carry on for almost a century.

Parts of the message disappear with time. DNA is packaged into chromosomes. Each has a specialised length of DNA at its end that marks the point at which the DNA-duplication machinery stops and loops back on itself, rather like the crimp at the ends of shoelaces that stops them from fraying. This gets shorter with age. In a baby it is about twenty thousand letters long, while in a sixty year-old it is less than half that length. Cells from tumours have lost even more DNA from the chromosome ends. About forty letters are dropped from this section of the message each time a cell divides, so that an old body works from an imperfect instruction manual, full of typographic errors. The same happens to mitochondria! genes, which are shot full of holes as rhe years conrinue their inexorable progress.