When President Bush signed the Genetic Information Non-discrimination Act (Gina) into law last month, it was hailed as the first civil rights legislation of the 21st century. The measure is designed to address one of the great fears of the genetic age - that, as we discover more about how DNA contributes to disease and behaviour, insurers might use the data to refuse cover to people with particular genes.

Individuals, of course, have no greater control over their genetic make-up than they do over their skin colour. The Act makes it illegal for US health insurers to ask potential clients to take DNA tests, or to demand the results of any that they have had already. Its aim is so honourable, and seems so fair, that it passed with barely a whisper of dissent. Just one congressman voted against, and it was backed unanimously by the Senate.

In the UK, the Association of British Insurers' decision last week to renew a moratorium on genetic testing, which will run until 2014, also met with near-universal approval. A consensus is forming that all genetic discrimination is unethical, and that action is needed to prevent the emergence of an uninsurable genetic underclass, denied mortgages, life and health policies through no fault of its own.

That consensus is wrong. It breaks with precedent, is unfair to businesses and many consumers, and imagines a threat to equality that is actually rather marginal, because of a misunderstanding of how DNA influences human health.

There is nothing new about genetic discrimination. It is enshrined in the insurance system, and accepted as entirely fair. Those irritating Sheila's Wheels adverts, selling women-only car policies, are genetic discrimination in action. They offer lower premiums to people who do not have a gene called SRY - the one that makes men male.

Insurers have also long been entitled to ask for a person's family history of disease before setting premiums. Yet family history is simply a less accurate proxy for genetic risk. DNA tests measure the same thing more precisely, and can open up insurance to some who are at present refused it.

People with a parent who suffers from Huntington's disease are often functionally uninsurable, as they have a 50 per cent chance of carrying a mutation that will kill them unless something else does first. Genetic tests can settle whether or not they have it, giving some access to policies they not have been denied before, while changing nothing for the unfortunate.

The non-disclosure of genetic information can cause other kinds of injustice. Mutual insurance works because both sides are equally blind to the future - neither insurer nor client knows who will have to make a claim and who will not. Yet if individuals discover a genetic predisposition to Alzheimer's or Parkinson's, which they do not have to reveal, they can play the system.

Research at Duke University has shown that people who learn that they have a genetic variant linked to Alzheimer's become more likely to buy long-term nursing care policies. That is perfectly rational, but is unfair not only to insurers, but to other clients, who must pick up the tab through higher premiums.

Perhaps the strongest argument against legislation, however, is that the realities of genetics generally make it unnecessary. Genetic determinism is often exaggerated: few disorders work like Huntington's, in which a single-gene mutation invariably causes a deadly disease. Common killers such as heart disease, diabetes and most cancers are influenced by DNA, but by dozens of common variants, each contributing only slightly to risk.

Few of these are known at present, so tests are poor predictors of health, and insurers would be unwise to rely on them. But even when the variants are more fully understood, the very fact that they are so common will limit discrimination. All people will carry some genes that predispose them to one condition and that protect them against others. My high genetic risk might be colon cancer, yours heart disease. There is no such thing as a perfect genome.

This means that insurance companies will rarely be able to pick and choose who to cover on the basis of genetic tests. They are businesses that need customers, all of whom have genetic flaws. They will have no option but to insure people with known risks, at acceptable premiums. Those that are too fussy will simply go bust.

The Government may need to make special arrangements for the unlucky few who have rare mutations that always cause disease - although, as these can usually be traced through family histories, this group is poorly served by the insurance industry at present. But for most of us, genetic testing is nothing to fear. The market can protect our civil rights as surely as the law.

Mark Henderson is science editor of The Times. His book, 50 Genetics Ideas You Really Need to Know, is to be published by Quercus

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