Genetics has at last provided the tools to test the pure race theory. The word 'race' itself is ill-defined. It includes social and political as well as biological criteria. In an attempt to escape a difficulty by renaming it the term 'ethnic group' is sometimes used. Such groups can define themselves. The Scots scarcely existed until they were invented by King George IV, who in i8zz visited Edinburgh and, dressed in a Stuart kilt and a pair of flesh-coloured tights, gave the Scots a national identity they never knew they possessed. It took only the imagination of Sir Walter Scott in devising a native culture to produce a new and potent myth. It was based on the kilt, which, as Macaulay said, 'before the Union, was considered by nine Scotchmen out of ten as the dress of a thief. The Celts, the larger unir to which the Scots claim allegiance, are themselves an illusion. Celtic culture, defined by artefacts excavated in southern Germany, was hijacked by the French and the Germans as well as the Celtic Fringe as a statement of national worth. In fact, trade had more to do with the spread of Celtic civilization than did sex or conquest.

For ethnic identity what matters is what group we think we belong to. For genes it is not so simple. Perhaps those that count are the ones most visible. After all, people do tend to choose mates of the same skin colour as themselves and this might be important when it comes to the nature of race. The theory of pure races made a definite claim about human groups; that they descended from a series of distinct ancestors. If this is so, and mere appearance represents the remnants of this history, then each race should be distinct in most genes and not just those for skin colour or hair form.

What does the genetic atlas look like? Are shifts in skin colour — the result of a dozen or so genes — matched by parallel trends in the tens of thousands of genes that build a man? The answer is, quite clearly, no.

Everyone can see global trends in colour, hair form and so on. Plenty of less obvious patterns exist but what is behind them is quite unknown. Some patterns are so obvious that they almost beg to be justified in selective terms. In England, the gene for blood group B is rare and is borne by fewer than one in ten people. In central Russia and in west Africa, in contrast, it is common, and up to a third of the population carry that variant. In the rhesus system a marriage between a positive man and a negative woman can be dangerous when the mother's blood reacts against that of her unborn child, but rhesus negative is common in Europe and Africa (albeit rare elsewhere). It must once have had some advantage that allowed it to spread in the face of this penalty.

Even the imaginative are pressed to explain some other trends in terms of selection. Most westerners have sticky ear wax, but that of most orientals is flaky and dry. And why can most Indians taste the bitter substance PROP while Africans cannot and what causes fingerprint patterns to vary so much across the world?

For none of these is there an explanation: but, as so mm. It ot modern medicine depends on genetics we have.irrived at the rather unexpected position of knowing more about the patterns of change in humans than in any other animal. Hundreds of functional genes and thousands of variants in the non-coding parts of DNA have been mapped. Most vary in frequency from place to place. The picture that emerges is quite different from that supported by those who believe mankind to be divided into distinct races. Man, it transpires, is the most boring of mammals, varying scarcely at all from place to place. The trends in physical appearance are not accompanied by those in other genes. Instead, the patterns of variation in each system are more or less independent. We would have a different view of race if we diagnosed it from blood groups, with an unlikely alliance between the Armenians and the Nigerians, who could despise the B-free people of Australia and Peru. Gene geography shows that people from different places do not differ much and that colour says little about what lies under the skin.

Imagine that the whole world could be measured for the diversity it contains. The job should be easy enough; after all, its people would all boil down into a soup which would just fill Windermere. The total set can be sorted out among individuals, countries and races to see how it splits up. The analysis, which is based on hundreds of genes in scores of populations, shows that around eight tenths of total diversity, worldwide, comes from the differences between the people of the same country: two Englishmen, say, or two Nigerians. Another five to ten per cent is due to the differences between nations; for example, the people of England and Spain, Nigeria and Kenya. The remainder — the overall genetic differences between 'races* (Africans and Europeans, for example) — is not much greater that between different countries within Europe or within Africa. DNA bears a simple message; that individuals are the repository of most variation. A race, as defined by skin colour, is no more an entity than is a nation, whose personality depends only on a brief shared history.

The notion that humanity is divided up into a series of distinct groups is wrong. The ancient private homeland in the Caucasus — the cradle of the white race — was a myth, as were its equivalents in Egypt or Peru. If, after a global disaster, just one group, the Albanians, the Papuans or the Senegalese, were to survive, most human diversity would be preserved. Humans are uniform creatures, because they evolved so recently. DNA sequence shows that the difference among the races is less than a fiftieth that between man and chimpanzee.

For other animals, race means more. The genetic differences between the snail populations of two adjacent Pyrenean valleys is far greater than that between Australian aboriginals and Europeans. For a snail it makes good biological sense to be a racist, but humans have to accept that they belong to a tediously homogenous species.

The fact that genes can be used to differentiate peoples {as skin colour does Africans and Europeans) is scarcely relevant to how different they are. After all, a forensic scientist can separate two brothers suspected of a crime with a blood sample, although the suspects share half their inheritance. Even a single gene may be a reliable indicator. If a bloodstain at the scene of a crime contains sickle-cell haemoglobin, it is almost certain that the suspect has African ancestry; but if it has the gene for cystic fibrosis {unknown among Africans) then the police should look  for a Kuropean. Neither observation changes the fact that Africans and Europeans have most of their genes in common.

The issue of differentiability versus difference causes controversy in the legal profession. DNA fingerprints are enormously variable. Confident claims were made about how they would revolutionise forensic science. In one American court, the prosecution described the chance of error as one in seven hundred and thirty eight million million. A single trace of DNA — blood, sperm, or even the saliva spat out onto the shirt of someone in close conversation with a supposed criminal — and the suspect would be identified. There was, it seemed, no room for argument. The case was so persuasive that sometimes judges even refused to hear evidence from the defence.

Now, life looks rather murkier. Of course, even if the test is infallible, the people who make it are not. There have been obvious lapses (such as mistakes in labelling samples). Other technical problems can also lead to difficulties.