Old age is itself in part the result of genetic accidents. Human cells in culture age more quickly when they carry a defect which increases the mutation rate and some children who inherit a tendency towards cancer also show symptoms of senility much earlier than normal. The immune system, which has the highest mutation rate of any part of the body, often fails as the years pass by. It seems that the decay of our elderly selves is, to some degree, a consequence of mutation. The influence of old age in damaging sperm and egg adds a certain irony to the claims of one institution devoted to the reversal of the decay of the human race, the Centre for Germinal Choice in California, in which Nobel Prize-winners make genetic deposits for hopeful mothers. The depositors may once, as they claim, have approached a genetic ideal, bur that perfection has been marred by age.

Why, if our genes change and decay through our lives, does the human race not degenerate as one generation succeeds another? The answer lies in sex. To define sex is simple; it is a process that brings together genes from different ancestors. It provides a chance to purge ourselves of the harmful mutations which arise in each generation and represents, in more ways than one, the antithesis of age.

Almost every novel, ptay or work of art revolves around the eternal triangle of sex, age and death. All three — and our very existence — emerge from errors in the transmission of genes. Humanity is not a degenerate remnant of a noble ancestor. Rather we are the products of evolution, a set of successful mistakes. Genetics has solved one of our oldest questions; why people decay, but Homo sapiens does not.

The plot of George Eliot's novel Daniel DcrotitLi is.1 convoluted one. It revolves around the adventures of himself, the adopted son of a baronet. After souk- hundreds of pages he develops an unexpected interest in things Hebrew and — some time later — it transpires that Daniel Deronda was, quite unaware, the son of a Jewish woman. His biology had triumphed over his background.

Many people are obsessed by the role of inheritance compared to that of experience. The infatuation goes back long before genetics. Even Shakespeare had a say: in The Tempest Prospero describes Caliban as "A devil, a born devil, on whose nature Nurture can never stick." There are still endless (and rather empty) discussions about whether musicality, criminality or intelligence is inherited or acquired and more serious debates about the role of genes and environment in illnesses such as cancer or heart disease. Such questions are often unresolved, and may be unresolvable.

Galton, in Hereditary Genius, went to great lengths to show that talent runs in families and was coded into their biology. He failed to point out that more than half his 'geniuses' turned up in families with no history of distinction at all and concentrated only on those who supported his hereditarian views. Most claims that talent (or lack of it) is inherited are based, like Galton's, on little more than a series of selected anecdotes. Even the descendants of Johann  Sebastian Bach disappeared from  the musical firmament after a few generations. Family likeness says little about the importance of biology; after all, one attribute much shared by parents and children is bank-balance.

Nevertheless, the question of nature versus nurture is of endless fascination. Dozens of studies purport to show that behaviour is under genetic control. Whole sets or degenerate families were once held up for inspection: the Tribe of Ishmael, the Jukes Clan and the Kalikaks (whose pseudonym is Greek doggerel for good/had). One was traced to an eighteenth century sailor who married an upright woman but had an affair with a slattern. His wife's branch gave rise to a lineage of spotless virtue while the other was a burden on society, as firm proof that morality lies in the genes.

Geneticists find queries about the importance of nature and nurture dull, for two reasons. First, they scarcely understand the inheritance of complex characters (those, like height, weight or behaviour which are measured rather than counted) even in simpler beings like flies or mice and even with traits which are easy to define. Second, and more important, geneticists know that the perpetual interrogation — gene or environment? — is often meaningless. Its only answer is that there is no valid question.

Although genetics is all about inheritance, inheritance is certainly not all about genetics. Almost every attribute involves the joint action of the internal and the external world. A characteristic such as intelligence (or height) is often seen as a cake ready to be sliced into so much 'nature' and so much 'nurture'. In fact, the two are so closely blended that to separate them is like trying to unbake the cake. A failure to understand this simple fact leads to confusion and worse.

Not far from Herbert Spencer's (and his neighbour Marx's) tomb is a large red-brick house. It was occupied by Sigmund Freud after he fled Austria to avoid racial policies which descended from the Galtonian ideal. On his desk is a set of stone axes and ancient figurines. Freud's interest in these lay in his belief that behaviour is controlled by biological history. Everyone, he thought, recapitulates in their childhood the phases experienced during evolution. Freud saw unhappiness as a sort of living fossil, the reappearance of ancient behaviour which is inappropriate today. Like Galton he saw the human condition.is formed by inheritance. The libido and ego.in¹, lu* wrote, ^lat bottom heritages, abbreviated recapitulations of the development which all mankind passed through from its primaeval days'. Freud hoped that once he had uncovered the inborn fault which caused despair, he might be able to cure it.

Today's Freudians have moved away from their guru's Gaitonising of behaviour. They feel that nurture is important. Analysis looks for childhood events rather than race memories. In so doing they are in as much danger as their master of trying to unbake the cake of human nature. Any attempt to do so is futile.

The Siamese cat shows how pointless the task may be. Siamese have black fur on the tips of the ears, the tail and the feet, but are white or light brown elsewhere. They carry the 'Himalayan' mutation, which is also found in rabbits and guinea pigs (but not, alas, in humans). Crosses show that a single gene that follows Mendel's laws is involved. At first sight, then, the Siamese cat's fur is set in its nature: if coat colour is controlled by just one gene then surely there is no room for nurture to play a part.

However, the Himalayan mutation is odd. The damaged gene cannot produce pigment at normal body temperature but works perfectly if it is kept cool. As a result, the colder parts of the cat's body, its ears, nose and tail (and, for a male, its testicles) are darker than the rest. An unusuallydark cat can be produced by keeping a typical Siamese in the cold and a light one by raising it in a warm room. Inside every Siamese is a black cat struggling to get out. To ask whether its pattern is due to gene or to environment means nothing. It results from both. What the Siamese cat — and every other creature- inherits is an ability to respond to the circumstances in which it is placed.