Genetics — science as a whole — owes its success to the fact that it is reductionist: that to understand a problem, it helps to break it down into its component parts. The human genome project marks the extreme application of such a view. The approach works well in biology as far as it goes, but it only goes so far. Its limits are seen in a phrase once notorious in British politics, the late Prime Minister Mrs Thatcher's statement that 'There is no such thing as society, there are only individuals.' The failures of her philosophy are all around us. To say, with Galton and his successors, 'There are no people, there are only genes* is to fall into the same trap.

In spite of the lessons of the past, there has been a resurgence of the dangerous and antique myth that biology can explain everything. Some have again begun to claim that we are controlled by our inheritance. They promote a kind of biological fatalism. Humanity, they say, is driven by its inheritance. The predicament of those who fail comes from their own weakness and has little to do with the rest of us. Such nouvelle Galtonism suggests that human existence is programmed and that, apart from a little selective pregnancy termination, there is no point in any attempt to change it — which is convenient for those who like things the way they are.

After the Second World War, genetics had — it seemed — at last begun to accept its. own limits and to escape its confines as the haunt of the obsessed. Most of those in the field today are cautious about claims that the essence of humanity lies in DNA. Although it can say extraordinary things about ourselves, gcm-iics is one of the few sciences that has reduced its expectations.

In mediaeval Japan, the science of dactylomancy — the interpretation of personality from fingerprints — had it that people with complex patterns were good craftsmen, those with many loops lacked perseverance, while those whose fingers carried an arched pattern were crude characters without mercy. Human genetics has escaped from its dactylomantic origins. The more we learn about inheritance the more it seems that there is to know. The shadow of eugenics has not yet disappeared but is fainter than it was. Now that genetics has matured as a subject it is beginning to reveal an extraordinary portrait of who we arc, what we were, and what we may become. This book is about what that picture contains.

The rich were the first" geneticists. For them, vaj;m* statements of inherited importance were not enough. They needed-and awarded themselves-concrete symbols ol wen It h.ind consequence that could persist when those who invented them were long dead. The Lion of the Hebrew Tribe of Judah was, until a few years ago, the symbol of the Emperor of Ethiopia, while those of England descend from the lions awarded to Geoffroy Plantagenet in i 177. The fetish for ancestry means that royal families are important in genetics (Prince Charles, for example, has 262,142. ancestors recorded on his pedigree). The obsession persists against all attempts to deny it. Heraldry was cut off by the American Revolution, but George Washington himself attempted to make a connection with the Washingtons of Northamptonshire and used, illegally, their five-pointed stars as a book plate.

Heraldic symbols were invented because only when the past is preserved does it make sense. For much of history wealth was dissipated on funerary ornaments to remind the unborn from whence they sprang. University College London contains an eccentric object; the stuffed body of the philosopher Jeremy Bentham (who was associated with the College at its foundation). Bentham hoped to start a fashion for such 'auto-icons' in the hope of reducing the cost of monuments to the deceased. It did not catch on, although the popularity of his corpse with visitors suggests that it ought to have done. Such pride in family would now be greeted, mainly, with derision. Harold Wilson, the British Prime Minister of the 1960s, did as much when he mocked his predecessor, Lord Home, for being the Seventeenth Earl of that name. Lord Home deflected the jest when he pointed out that his critic must be the seventeenth Mr Wilson. He made a valid claim: that while only a few preserve their heritage in an ostentatious way, every family, aristocratic or not, retains the record of their ancestors. Everyone, however deficient in history, can decipher their past in the narrative of the DNA.'

Some can use inherited abnormalities. A form of juvenile blindness called hereditary glaucoma is found in France. Parish records show that most cases descend from a couple who lived in the village of Wicrr-Effroy near Calais in the fifteenth century. Even today pilgrims pray in the village church of Sainte Godeleinc, which contains a cistern whose waters are believed to cure blindness. Thirty thousand descendants have been traced and for many the diagnosis of the disease was their first clue about where their ancestors came from and who their relatives might be. The gene went with French emigrants to the New World.

Human genetics was, until recently, restricted to studying pedigrees that stood out because they contained an inborn disease. Its ability to trace descent was limited to those few kindreds who appear to deviate from some perfect form. Biology has now shown that perfection is a mirage and that, instead, variation rules. Thousands of characters — normal diversity, not diseases — distinguish each nation, each family and each person. Everyone alive today is different from everyone who ever has lived or ever will live. Such variation can be used to look at shared ancestry in any lineage, healthy or ill, aristocratic or plebeian. Every modern gene brings clues from parents and l',r: inilp.imiis, from the earliest humans a hundred thousand years and more ago and from the origin of life four thousand million years before that.

Most of genetics is no more than a search for diversity. Some differences can be seen with the naked eye. Others need the most sophisticated methods of molecular biology. As a sample of how different each individual is we can glance beneath the way we look to ask about variation in how we sense the world and how the world perceives us.

Obviously, people do not much resemble each other. The inheritance of appearance is not simple. Eye colour depends first on whether any pigment is present. If none is made the eye is pale blue. Other lints vary in the amounts of the pigment made by several distinct genes, so that colour is not a dependable way of working out who fathered a particular child. The inheritance of hair type is also rather complex. Apart from very blonde or very red hair, the genetics of the rest of the range is confused and is further complicated by the effects of age and exposure to the sun.

Even a trivial test shows that individuals differ in other ways. Stick your tongue out. Can you roll it into a tube? About half those of European descent can and half cannot. Clasp your hands together. Which thumb is on top? Again, about half the population folds the left thumb above the right and about half do it the other way. These attributes run in families but their inheritance, like that of physical appearance, is uncertain.

People vary not just in the way the world sees them, but how they see it. A few are colour-blind. They lack a receptor for red, green or blue light. All three are needed to perceive the full range of colour. The absence of (or damage to) one (usually that for green, less often for red, almost never for blue) gives rise to a mild disability that may have made a difference when gathering food in ancient times. The three genes involved have now been tracked down. Those for red and green are similar and diverged not long ago, while the blue receptor has an identity of its own. John Dalton, best known for his atomic theory, was himself so colour-blind as to match red sealing-wax with a leaf (which must have made things difficult for a chemist). He believed that his own eyes were tinted with a blue filter and asked that they be examined after his death. They were, and no filter was found, but, a century and a half later, a check of the DNA in his pickled eyeballs showed him to have lacked the green-sensitive pigment.