Cloning, though, is but the latest stage in the manipulation of our reproductive machinery. Most people are ready to accept pregnancy termination on genetic grounds; and, in spite of the concerns about modified plants and animals, have no complaints about gene therapy, should that ever come to fruition. However, to interfere with the next generation, by engineering eggs or sperm or by cloning is, it seems, a step too far.

Even before Dolly, sexual technology had begun to explode. The demand is high. One married couple in six suffers from some failure of fertility, and miscarriages take place in about the same proportion of all pregnancies. At least a million people have been born by artificial insemination, and by 2005 there may be almost as many who trace their origins to a test-tube. Indeed, the chances of reproductive success in such a vessel are higher than those when trying to have a baby by more traditional means. Now, such methods are beginning to change the practice of genetics.

Many inborn illnesses, from PKU to cystic fibrosis, can be treated with some success. Such treatments deal with symptoms, rather than putting right the fundamental flaw — which is no more that what medicine does for most illnesses. Gene therapy gives hope of a cure. In its purest form, it offers the hope of replacing a faulty section of DNA with a normal equivalent, and putting right the problem at source. The idea is a powerful one: and nobody who accepts the necessity of inserting a new heart and lung can quarrel with the idea of replacing a piece of nucleic acid. Whatever its promise, gene therapy has, unfortunately, failed to live up to its headlines.

In principle, the job ought to be if not easy, at least feasible. DNA can be inserted into cells in culture in many ways. Copies made in a laboratory are infiltrated with the help of a virus or wrapped in envelopes of fat that are accepted by the cell as its own. Working genes can even be shot into cells by firing gold spheres coated with DNA from a tiny gun. Twenty years ago there were great hopes that such technology would revolutionise medicine. There have been many claims of success, but only one has much weight. Severe combined immunodeficiency is an inherited failure of the immune system that arises from the absence of a certain enzyme. Children with the condition are kept in a plastic bubble to reduce the chances of infection, and are given bone marrow transplants and injections of the enzyme to help their defences. Cells which lack the crucial protein have been 'cured¹ with the appropriate DNA. Several children have been treated with such engineered cells- They are still alive and even go to school. As most of them were also given extracts of the enzyme it is not yet certain that their improved health is due to the gene manipulation.

Whatever this success, all other claims have been premature. Two hundred or so patients with cystic fibrosis have been treated and the best result has been a small and transient improvement in symptoms. The technology has risks of its own. One American patient with liver disease was injected with an engineered virus based on that for the common cold and died as a result. Many others in the trials have not survived (although most were already desperately ill). Some of the most frequent diseases are going to be difficult to treat. To cure sickle-cell would involve targeting tiny numbers of cells deep within the bone marrow, as it is these and not the red blood cells which produce the faulty haemoglobin. For diseases such as muscular dystrophy it might be necessary to deliver a gene direct to millions of separate muscle cells, and to ensure that it is switched on at just the right level of activity.

Molecular biology could be used in medicine in many other ways, some of which have attracted the 'gene therapy' label. Cells can be engineered to carry genes that destroy cancer cells or cause them to stop dividing. It might be possible to introduce DNA that stimulate the immune system's own defences inro cancer cells themselves, providing them with the seeds of their own destruction. Another ingenious idea is to insert drug-metabolising genes into such cells, and then to treat them with a chemical that is broken down into a poison — but only in cancer cells. To establish the DNA sequence of a faulty gene also gives the prospect of making 'anti-sense' nucleic acid which binds to the genetic message and blocks it to turn off genes that have gone wrong. All this lies in the future.

The new biology offers more hope for improvements in diagnosis. Molecular probes can detect mutations long before symptoms first appear. Cancer cells often develop unusual antigens on their surface as new genes are switched on. It may become possible to work out the shape of the protein involved and to make a match that sticks to the relevant place. Not only will this show where the damage lies, but if a drug is attached, it may be possible to point a treatment straight at its target.

Engineering might do even more: in theory it could be used to treat generations yet unborn. In mice this has already succeeded. Genes inserted into sperm or egg cells may be passed on. The germ line, as it is known, has been changed. Such 'transgenic mice' are valuable research tools. If genes for a human disease are introduced they can used to study its symptoms (although these may differ those found in humans themselves) and the mice may be used to test drugs. Transgenic mice have been made for sickle-cell anaemia and other inherited illnesses, as have transgenic pigs with some of the genes for human cell surface variation. Their organs — heart and kidney — are about the right size for a transplant and are more acceptable to a human recipient than they otherwise would be. Such pigs look just like pigs; but, to our immune system, resemble a human being. A counterfeit heart has not yet been used for transplantation, but soon may be. As more than a hundred and fifty thousand people die in Britain each year because it is impossible to find a matching organ this may become important in medicine.

Every therapy must work to rules. Everyone has rights to their own body and can decide whether or not to accept treatment. The same logic can be applied to genes. To replace damaged DNA, should that become possible, is not much different from transplanting a kidney and the same choices must be made by the person who receives it. To change genes in sperm or egg is different because it alters the inheritance of someone who has no choice. Many feel that on this and other grounds germ line therapy is unacceptable and have tried to add to the Universal Declaration of Human Rights a statement that everyone has the right to a genetic constitution that has not been changed.

Given that any medical advance is likely to alter the genes of future generations that seems a little too inclusive; and the failure of gene therapy puts the notion for the time being in the field of speculation. However, another set of technologies which once seemed impossible has — in contrast to gene manipulation itself- turned out to be remarkably simple. Their potential use on humans has caused a storm; but that is nothing new.

Genetics outside the uterus allows reproduction to be controlled. It ranges from artificial insemination to surrogate motherhood to germ-line therapy to cloning. Each one was, on its introduction, greeted with horror (and British children born by donor insemination were once defined to be illegitimate because of the objections by the bishops) but most in the end were accepted. However, as Raskolnikov puts it in Crime and Punishment: 'Man gets used to everything — the beast!' Philosophers talk of the 'yuk factor', the automatic revulsion about interfering with our reproduction. Forty years ago it was, on just those grounds, illegal in Britain to save eyesight by grafting on a cornea from a dead person. Philosophy, it seems, is not much help to the blind.