Nature versus nurture

But even if it does eventually lead to an understanding of the function of every gene, will it really allow us to accurately predict who will develop heart disease, become violent or become homosexual?

What about our childhood, our diet, our living conditions, our stress levels? Will scientists still need to look to our environment to explain our health and behaviour?

Of course, say gene experts. But intriguingly, the revelation of the role of genes could lay bare the influence of environmental factors with greater clarity than ever.

Dr Paul Kelly, director of Gemini Genomics in Cambridge, UK, says the sequencing "will make environment more important, not less important".

Identifying factors

For example, by studying the DNA of someone suffering from diabetes, scientists will be able to work out to what extent their genes are responsible for their disease, and to what extent their lifestyle is the problem.

This will allow a doctor to tailor a treatment that might involve only specific lifestyle changes.

Professor Peter Little, of Imperial College, London, says: "By understanding genetic changes, we can then go on to identify the environmental changes that contribute.

"If we can identify the nature, which we will do to a very significant extent, we can use this to identify the nurture. This is the second goal of the Human Genome Project."

But anxiety remains that the hype surrounding the sequencing of DNA will overwhelm a more balanced view of what makes Homo sapiens human.

Complex traits

"It's a concern that the importance of environmental influences will be lost in the fanfare about genetics," says Professor Robert Plomin at the Institute of Psychiatry in London.

"The first message from genetic research is that genes play a surprisingly important role for almost all complex traits, whether behavioural or medical.

"But the second message is just as important: individual differences in complex traits are due at least as much to environmental influences as they are to genetic influences."

In other words, though gene disorders are the sole cause of a small minority of diseases (e.g. haemophilia), the genetic component of the cause of most diseases could be no more than 50%.

The other 50% might be down to our environment, which may involve experiences in the womb, as well as after birth.

And environmental influences may be at least as important as genetic factors in determining intelligence and other aspects of personality.

By Emma Young