Genetics Today

Genetics began by being ignored. Now it has the opposite problem. Mendel was dismissed because his work seemed unimportant, but today genes are everywhere and the public is fascinated by their promises and disturbed by their threats. Scientists have been quick to emphasize both. Not for nothing has it been said that the four letters of the genetic code have become H, Y, P and E.

The last decade's advances have been amazing. We have the complete sequence of the DNA letters of the 60,000 or so working genes needed to make a human being, and will soon have that of all the so-called "junk" DNA sequence (which may reveal that it does more than its name implies). 10,000 different diseases have an inherited component, and - in principle at least - we know the genes involved.

That raises both hopes and fears. For diseases controlled by single genes, such as sickle-cell anaemia or cystic fibrosis, it has become easier to identify both carriers and foetuses at risk. Because any gene can be damaged in many ways - for example, there are more than 1,000 known mutations for cystic fibrosis - the tests are not straightforward, and often the best that will be possible is to tell people that they are carriers, rather than to reassure them that they are not. The decisions as to whether to become pregnant or to continue with a pregnancy will, however, become somewhat easier as the tests become less ambiguous.

Tests are commercially available for genes predisposing to cystic fibrosis and breast cancer; and the development of DNA "chips" that can screen many genes at once means that more will soon be on sale. Medicine will have to deal more and more with those who have - rightly or wrongly - diagnosed themselves as at risk.

Most people, we now realize, die of a genetic disease, or at least of a disease with a genetic component. For some, it will become possible to tell them of their plight - but why should we want to do so? Sometimes, the information is helpful. Those who inherit a disposition towards certain forms of colon cancer, for example, can be helped by surgery long before the disease appears. For other illnesses, people at high risk can be warned to avoid an environment dangerous to them. Smoking is dangerous, but a few smokers get away with it. However, anyone who carries a changed form of an enzyme involved in clearing mucus from the lungs will certainly drown in their own spit if they smoke - and that might be enough to persuade them not to. However, knowledge can be dangerous, particularly when health insurance gets involved.

The most successful kind of medicine has always been prevention rather than cure. Genetics is no different, and the hope of replacing damaged DNA by gene therapy is still around the corner, where it has been for the past ten years. Genetic surgery - the ability to snip out pieces of DNA and move them to new places - has done remarkable things, but so far has done little to cure disease.

It might, though, help prevent the world's population from starving, at least according to enthusiasts for genetically modified (GM) foods. They may be right. It has proved remarkably easy to move plant genes around. Already there are crops that have been altered to make them resistant to parasites, or to artificial weedkillers (which means that the fields can be sprayed, leaving the crop unharmed). Commercial optimism has, in Europe if not the United States, been matched by public concerns about health risks. Why people are worried by the remote risk that GM foods might be dangerous to eat when they are happy to eat cheeseburgers that definitely are, mystifies scientists, but science is less important than what consumers are willing to accept. Unless attitudes change, the hope of putting genes for, say, essential nutrients into Third World crops will probably not be fulfilled.

If interfering with plants alarms society, to do the same with animals outrages a vocal part of it. We still know rather little about how a fertilized egg turns into an adult, with hundreds of different kinds of tissue, each bearing exactly the same genetic message but with jobs as different as brain cells and bone. Although it has long been possible to grow adult plants and even frogs from single cells, the notion that it might be possible to do so with mammals seemed a fantasy - until the birth of Dolly the sheep in 1997. Then, with the simple trick of inserting the nucleus from an adult cell into an emptied egg and allowing it to develop inside a foster-mother, a sheep was made without sex: it was cloned.

Cloned sheep or cows might be important in farming, and might be used to make multiple copies of animals with inserted human genes for proteins such as growth hormone (which are already used in "pharming", the production of valuable drugs in milk). The publicity that followed Dolly led to immediate condemnation of the idea of human cloning, often without much thought as to quite why it should be so horrific. After all, we are used to identical twins (who are clones of each other), so why should an artificial version cause such horror? In the end, again, public opinion moulds what science can do, and the prospect of cloning a human being seems remote.

And why might anyone want to do it? Claims of an army of identical Saddam Husseins verge on the silly, and others of replicating a loved child who died young also seem unlikely. However, the technique has great promise in medicine. Cells of the very early embryo (stem cells, as they are called) have the potential to divide into a variety of tissues, and can be grown - cloned - in the laboratory, or even manipulated with foreign genes. Perhaps they could make new skin or blood cells, or, in time, even whole organs. Because this involves the use of very early embryos, made perhaps by artificial fertilization in the laboratory and not needed for implantation into a mother, it has become mixed up with the abortion debate. In the United States, the "Pro-Life" lobby has succeeded in denying funds from government sources for such work.

Genetics is always mixed up with politics. It has been used both to blame and to excuse human behaviour. The claim (in the end not confirmed) of a "gay gene" led to two distinct responses among the homosexual community. Some feared that the gene would be used to stigmatize them, but most welcomed the idea that their behaviour might be coded into DNA, as it meant that they could not be accused of corrupting those not already "at risk". Such opposing views apply just as much to the supposed genes that predispose to crime - are they evidence that the criminal cannot be reformed and must be locked away for ever, or should they be used in mitigation to argue that he was not acting according to his own free will?

Science has no answer to such questions, and in the end the most surprising result of the new genetics may be how little it tells us about ourselves.