Know your own mind; UNDER THE MICROSCOPE

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Within the last decade the concepts of cloning; genetic engineering; and the employability of the molecular biologist; have all permeated the consciousness of even the least technologically orientated. Swept up in this enthusiasm to track down the genetic basis of everything, it is easy to slide into blaming all of our less desirable quirks on the DNA capriciously dealt out to us by an oblivious mum and dad. Everyone has heard of the suggestions of genes for gayness and criminality - are we merely hapless victims of those twisted strands of nucleic acid? Is everything, not just eye colour, but short temper, irrefutably "genetic"?

Of course not. In the film, The Boys from Brazil, clones of Hitler were being reared in different countries. With great contrivance, each boy was subjected to life experiences similar to those of their dead model in an attempt to recreate, with chilling precision, a future Fuhrer. Why such bother? In one sense we are no more and no less than our genes dictate. All we are are cells, and each cell has its genetic blueprint. But the whole is more than the sum of its parts, never more so than in the brain. As our brains become more complex, the nature-nurture seesaw starts to rock into play.

The critical question is what exactly an individual gene might do. This problem is particularly acute in relation to the functioning of the brain. Of course, a gene can contribute to a certain event or behaviour, but surely it is not always an inevitable and relentless one-to-one correspondence with some macro characteristic, feature or action. Indeed, the idea of a single gene for a complex character trait has a trace of the flavour of a doctrine popular in the previous century, that of phrenology. The phrenologists happily pointed to a certain lump on the skull and matched it up with sophisticated nuances of character such as "love of country" or "banality". In such a scheme identical twins would not have merely similar proclivities - interlaced as they tend to be with curiously divergent characteristics - they really would behave as clones. The Boys from Brazil would have had no storyline.

Goldfish and flies may well behave like clones. Their genetic instructions for life allow for little deviation from the species script: the interactions with the environment are predictable. As humans enjoy increasingly fancy brains, however, childhood becomes longer, and the advantages of an individual interaction with the world greater. The effects of the environment on the brain have been long appreciated, even in rats. Merely allowing rats to interact with toys, for example, will be manifested as a greater number of connections between neurons, compared to their simply housed litter mates. How much greater for us humans, therefore, for whom connections, not neurons, account for the astonishing growth of the brain after birth. Our chimpanzee cousins, by comparison, born with a brain size similar to ours, undergo no such expansion. Versatile though the repertoire of a chimpanzee might be, we humans are able to learn still more for ourselves, and hence have even still greater scope for individuality.

Genes play an undeniable role in what we will do and be, but the environment must not be forgotten. In the US, a worrying system of "hot-housing" children - who at a few years of age can play the violin and speak Japanese - speaks for the need of the brain to realise its awesome yet malleable potential through external stimulation. Although everyone is aware of an obvious influence of the environment, the danger is that we might start to abandon the idea of self-determination and accountability.

In previous centuries people grew up with the idea that they had to know their place: their functions in life were preordained by a rigid social system, ostensibly by a God who "made them high and lowly, and ordered their estate". How unfortunate if science has now sown the seed that one is after all ordered, this time by DNA. As brains become ever more complex in the higher reaches of the animal kingdom, not justsurvival value, but outlook and performance, is increasingly rooted in the particular experiences of a particular lifetime. The individual brain is forged from an intriguing yet incompletely understood interaction of environment and genes.

Even when a problem is clearly "genetic", the causality is far from a phrenology-like correspondence of molecule to function. The challenge is to know not just that a gene is associated with a certain disorder, but exactly why and how an aberration in gene expression at the level of an ion channel, a chemical messenger, or even a neuronal circuit should be translated into a malfunction of movement, sensation or thought. The astonishing advances in genetic engineering and the awesome promise of gene therapy should not blind us to the power of the environment, the as yet intractable complexity of the brain, and the emergence of the individual mind.

Susan Greenfield is a neuroscientist at the University of Oxford, and Gresham Professor of Physic, London