Steve Jones, the truculent professor of genetics at University College London, says the four letters of the DNA code ought to have been H-Y-P-E. "The promises that have been made over the past 10 to 15 years about how this was going to change everything - it really has been a disgrace," he snorts.

It has been suggested, for example, that the discovery that humans share 98 per cent of their DNA with chimpanzees could radically alter mankind's view of itself. Prof Jones counters that if biology has changed human self-perception at all, it did so not in 2000, when a draft of the human genome was sequenced, but in 1859 when Darwin's On the Origin of Species first set the world ablaze.

"The idea that humans are just shaved monkeys has been around since Victorian times," he says. "Everything since then is merely detail."

And yet last year's announcement that scientists in the private and public sectors had sequenced 3bn pairs of chemical letters - the Cs, Gs, As and Ts that are the code for human life - was compared with everything from the US moon landing to the discovery of fire.

Mike Dexter, director of the Wellcome Trust, the UK charity that played a big role in the public sequencing effort, says the code will have more lasting significance than the invention of the wheel. "I can well imagine technology making the wheel obsolete. But this code is the essence of mankind, and as long as humans exist this information is going to be important," he says.

Matt Ridley, author of Genome, says genomics will have far-reaching implications, though probably not as soon as people imagine. Paraphrasing Arthur C Clarke, he says: "We always tend to over-estimate the impact of technology in the short-term and underestimate it in the long-term."

Mr Ridley adds: "There has definitely been a lot of hype around. In a few years we are not going to see a great shower of genome-based drugs. In fact, we'll see a shower of articles asking: 'What has happened?'"

But in 10 to 20 years, he continues, it will probably be routine for doctors to test the genotype of individual patients to find which particular gene has gone wrong. Instead of diagnosing, say, heart disease, a cardiac condition might be categorised - and treated - according to its underlying genetic cause.

Many of today's drugs work only on a subset of the population, with the rest being non-responders. In some patients, a particular drug's benefits may be outweighed by its side effects. Gene-based diagnostic tests should be able to sift out these reactions in advance.

Ironically, says Mr Ridley, the more sophisticated the diagnostic, the greater the potential for low-tech solutions. Patients may be advised to change their diet, for instance, perhaps based on their ability, or inability, to metabolise certain food types.

Prof Jones, whose books on the subject include The Language of Genes, says it will become increasingly less useful to distinguish between the environmental and genetic causes of disease. "The question of whether or not an illness is genetic is academic because we know that all diseases are genetic - or at least have a genetic component."

For example, he argues, if everyone smoked, then smoking-related illness would be regarded as genetic, since some people are better-equipped genetically to deal with poisonous nicotine. Humans are "not a cake sliced into a piece called nature and a piece called nurture", he says.

Because treatments are likely to trail diagnostics by several years, if not decades, Mr Ridley says the initial medical applications of genomics could be limited. What use is it to know you have a greater likelihood - even a certainty - of succumbing to a particular disease if there is nothing you can do about it?

Nancy Wexler, who discovered the gene that causes the untreatable Huntingdon's disease, decided not to take her own diagnostic even though she has a 50 per cent chance of inheriting the condition. To know, she reasoned, would be to play the part of Tiresias, the blind seer of Thebes who could foresee calamities but had no power to halt them.

And what if one can act? A rash of women who have had precautionary mastectomies after being told they are genetically predisposed - but not predetermined - to get breast cancer has led many to question the value of ambiguous diagnostic tests.

What humanity does with its genetic knowledge will be one of the great questions of the century. Lee Silver, professor of molecular biology at Princeton, has argued that it is impossible to halt the application of science in a market economy.

As scientists gain greater understanding of how to confer advantage on new-born babies - by selecting which embryos to implant or, eventually, by adding genes carrying traits such as "intelligence" - societal pressure will grow to ensure one's offspring is part of the genetic elite, he says. Eventually, he envisions a society of irrevocably divided between "natural" humans and a gene-rich superclass.

Mr Ridley agrees that a minority will be tempted to improve their genetic stock. "At the moment it seems that humans are more willing to contemplate the genetic engineering of ourselves than they are the genetic engineering of soyabeans," he says. "You can imagine there are people who will want to go beyond curing to enhancing."

For those who find this possibility terrifying and distasteful, there is hope. The sheer complexity and interdependence of the thousands of genes that make up human life may render it impossible to influence anything but the most basic traits. For centuries to come, the best way of confering "intelligence" upon one's offspring will still be by sending them to the best schools.

Far from leading us into a genetically deterministic nightmare, knowledge of the human genome may actually teach us to look afresh at the diversity and richness of human life. Just as no one complains that all Shakespeare's works are made from a simple 26-letter alphabet, so we need not worry that human life is written in a four-letter code.

Humanity's great advances have been social and organisational, not biological, says Prof Jones. "We stopped evolving 100,000 years ago, as soon as we started talking," he says.

"Civilisation has put biology on the back burner."