It may be unnatural, but encouraging genetic choice in humans is not bad. (It's also inevitable.)

We might as well start with Dolly. The past five years have seen plenty of other breakthroughs, from bacterial genome sequences to headless frogs, but none has had quite the same impact as the little woolly clone. Maybe that's because, in an odd way, we were prepared for her. Unlike homeotic genes, or DNA hybridization arrays, or secondary cellular messengers, or most of the rest of the stuff of the new biology, cloning is something we think we understand. And there's something deeply emblematic about it, too. Inasmuch as she's just another sheep, Dolly's completely natural; inasmuch as she's a clone of one particular other sheep, she's utterly unnatural. So there she stands, nature and artifice wrapped up into one bundle. Welcome to the future. Ecce ovo. Baaa.

The fact that she was a clone was actually not the most interesting thing about Dolly. Being able to xerox single creatures is not a particularly interesting skill in most circumstances. The important thing about Dolly is that a nucleus was taken from a cell in a laboratory test tube and put into an egg. Manipulating the genes of cells in laboratories is something molecular biologists are getting better and better at. The Dolly technique means that these manipulated genes can now be slotted into eggs much more precisely. From the laboratory and the clinic, the new biology is now poised to move into the field, or the barnyard, or the home. The first practical applications will be sheep with valuable proteins in their milk. Then there may be pigs that make organs for transplanting into humans. Then pets with engaging predetermined characters and advertising logos growing in their fur.

The first of those two applications is pretty widely acceptable. The second is still disgusting to many. It falls prey to what Tom Wilkie, who runs the biomedical ethics section of the Wellcome Trust in London, calls "the Yuk factor." The Yuk factor governs the initial public response to almost every biomedical advance that can easily be understood as being unnatural. Women giving birth after 60? Yuk! Pigs as organ donors? Yuk!

The Yuk factor feels instinctual, primal, a law of nature. Yet it can pass quite quickly. Take the cornea. In the 1950s it was against the law in Britain to save a patient's sight by grafting a dead person's corneas onto the patient's eyes. This was not just a legal oversight; people found the idea quite deeply Yuk, and it took a prolonged journalistic campaign to get the law changed. (Every contact lens wearer goes through the same process on a personal level. At first there is a definite Yuk to touching the eye; soon it becomes utterly commonplace.)

The Yuk factor boils down to a disgust at what seems unnatural. As we live with the unnatural, though, we begin first to accommodate it, then to accept it, then to appreciate it. That's just as well, because our success as a species rests on ever greater unnaturalness. Sheep were a product of our willingness to disregard nature long before we started cloning them, bred as they were for wool, for mutton, for a willingness to be herded. And what we do to sheep is nothing compared to what we're willing to do to ourselves with the help of qualified doctors and pharmaceutical prowess. The Yuks that surround the new biology reflect its ability to take our unnaturalness to hitherto impossible heights; but our enthusiasm for medicine will overcome them.

Biology, after all, is about life, not nature. It's just an accident of history that, until recently, everything alive was more or less natural. Nature is a record of 4 billion years of life's successes, written in the language of the genes. Biology's new strength comes from being able to read that record; cracking the genetic code has ushered in one of those wonderful eras of scientific progress when new discoveries keep leading to new techniques with which yet more discoveries can be made. From the ability to splice genes comes the monoclonal antibodies that recognize proteins, the DNA amplifiers that pick up genetic signals previously inaudible, the probes that tell one gene from another, the techniques for making mutations ever more subtle and specific. What used to be a Nobel Prize-worth of research is now a couple of months of PhD drudgery; what PhDs used to do with pride is now done by robots with efficiency. The past five years have seen the knowledge machines blur into ever faster productivity.

That knowledge is beginning to be put to use in unnatural wonders all around the world. There's Dolly. There are millions of other genetically engineered animals, and countless billions of cells in culture, some of them pumping out life-giving medicines under the auspices of biotechnology companies. There's a young woman named Louise Brown, and thousands of children created through in vitro fertilization after her. There's a girl called Ashanti DeSilva who was the first child to be deliberately infected with a virus that was supposed to knit a new gene into her cells. And this is all just the beginning.

It's unnatural; it's Yuk; but it is not bad. The natural has no special moral status; it merely has a practical pedigree. That which is natural has the advantage of having been shown to work, and we should bear that in mind. But no gene ever knew what would work in advance, or applied itself to a greater purpose than its own replication; the choices recorded in the genome are not moral choices. Morality has only now come to the genome, because only now is the genome open to deliberate action by people with foresight and responsibility. We can choose life in ways nature could not. But we should not be bound by it.

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Humanity is used to power over nature. Physics has given us abilities that most of our ancestors would have reserved for the gods; the power to visit new worlds, to end this one, to see everything, and to be heard everywhere. All this in barely a hundred years. Biological power will bring changes as profound - and choices with consequences as grave. Unfortunately, most people do not understand that these choices are becoming increasingly possible; they see nature as a set of bonds they cannot break.

The material basis of human nature is more widely accepted today than ever before, largely due to the new biology. The mind has been dissected; thoughts have been imaged; moods are altered. At an intellectual level, human nature, long exiled by social theorists, has made a comeback. To think of the mind as an evolved organ, its patterns of thought as biological as the metabolic pathways of the liver, is now to be at the vanguard of what is called evolutionary psychology, a field that over the past five years has grown from the obsession of a few to the intellectual fashion of many. While its findings may sometimes be fallacious, its foundations are clearly true: If the mind was not shaped by God, then it was shaped by evolution and culture.

Add to this a misleading way of talking about genetics that involves tagging genes with the problem that accompanies their dysfunction - genes "for" obesity, aggression, dyslexia, and addiction, and their more obviously medical cousins, the genes "for" cancer, Alzheimer's, and high cholesterol - and you get a world where people begin to think that their genes hold road plans for their lives. That their nature controls them.

But the organism is not specified by its genes as a car is specified by its blueprints; it is always already a work in progress. A living being is an information processor that continuously remakes itself, an interaction between matter and information in which neither can take priority. It is a dance of form and substance, of nature and nurture, of matter and information.

At the moment we know a lot about data in our genes, because we recently learned how to read it. We know far less about the processes that use that data to make cells, organs, behaviors, peo-ple. That means that, at the moment, people can be faced with the dilemma of knowledge that brings with it little power; they can know they face an increased risk of breast cancer thanks to a faulty gene, thanks to a recently developed test, but not know what to do with that knowledge.

This is a temporary problem. There is every reason to think that we can develop ever subtler ways to change the environment - the social environment, the global environment, the environment in our guts or our blood or our brainstem. And so we can change the way the genetic information is processed.

In some cases we already know how to do this. There is a genetic defect called phenylketonuria (PKU) that disrupts the ability to metabolize the amino acid phenylalanine. Left to itself, it leads to severe mental retardation; in that much, it is a gene "for" imbecility. These days, though, it is not left to itself. In America and much of the rest of the world, every newborn child is tested for PKU, and those who have it are then put on a special diet, one low in phenylalanine. The diet is not pleasant, and sticking with it is hard (it may become easier when cows are engineered to produce phenylalanine-free milk, a development project already under way). But stick with the diet and the child can develop pretty normally. Knowing about nature lets you change nurture to match. The two can be made to mesh.

PKU is a somewhat simple example. But the principle it embod-ies is one that should guide us through all the new knowledge and pseudo-knowledge about human nature and the workings of the mind that biology is producing at ever greater rates. Knowing the score is the first step toward putting the process back on the rails. Knowledge about genes is the beginning, not the end - the point from which you can start making choices about how to shape your world.

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Addressing a conference at Caltech a few years ago, physicist and science fiction author Gregory Benford imagined the perplexity of historians 50 years hence. "They'll look back at our abortion debates and they'll laugh; they won't see how we could get so worked up about such a simple choice when their biological choices are far, far more complex."

Perhaps the biggest choices will be about children. Better environments tailored for their genes are one thing; but what about better genes, too? Wholesale engineering of human children seems unlikely, for a range of practical, social, and ethical reasons to do with creating human life as an experiment. Much more likely is widespread "genotype choice" - a sort of decentralized, do-it-yourself eugenics.

A couple goes to a clinic and provides some sperm and some eggs. The clinic turns them into embryos and analyzes the different mixtures of the parents' genes each embryo carries. The parents are given the embryos' genetic profiles and advice on how the genes relate to various traits, both physical and mental, in various different conditions. At present, such a profile would be expensive and crude, capable of spotting genes for serious genetic disabilities but not much more. But with better DNA-analysis tools and much more knowledge about which genes do what - both fields that are growing exponentially - the pictures will get sharper and sharper. The parents choose the profile they like, on whatever criteria appeal to them; the chosen embryo is grown a bit further in the test tube, a few cells are snipped out to provide tissue for repairs in later life, and then the pregnancy gets under way. No engineering; just choice.

Many people, including some of the scientists who have made it possible, see this as unethical, the creation of a life as a commodity. Yet people create lives for self-centered purposes - such as support in old age - all the time. Genotype choice would undoubtedly be deeply unnatural. But so is birth in a hospital, not to mention contraception. It would give parents a real power over the sort of people their children will turn out to be. But parents have that power already, to a large degree - through attitudes, affection, and school choice.

Eugenics - the favoring of the reproduction of some genes over others - has a vile history. If genotype choice were to be in the hands of the state, it would likely be a continuation of that history, and a thing to fear and reject. But there is no need for it to be in the hands of the state, and every reason to fight against any strict control the state might try to exercise. Genotype choice should be a matter for individuals, a personal choice about what sort of life they want their children to have.

There would be effects beyond the level of the individual in such a world, as various traits became more common. Symmetrical features would seem a good bet, along with strong immune systems and some types of intelligence. Judging by personal ads, a gene for a good sense of humor would spread like wildfire. Severe genetic disorders would disappear. None of these is clearly a bad thing. Perhaps the most worrying potential by-product of genotype choice would be a skewed sex ratio - and that is already upon us. In India, China, and other countries, ultrasound imaging has led to the preferential abortion of female fetuses on quite a large scale. What can be done about this, though, is not clear. Sex tests are easy enough for a black market to spring up easily; so is abortion. It may be that the only thing to do is to proselytize for women's rights, to convince people that a girl is as valuable as a boy. And to remember that over time, if sex selection swings a long way in one direction, it is bound to swing back.

The other major social impact of genotype choice is far further off: longer lives. Aging research has already become a hot topic; the boomer generation that grew up with routine vaccination and with drugs that actually worked has started to face mortality. This is a huge constituency for advanced medical technology. A few years ago, a biotech lobbyist told me that every conversation he has with a legislator sooner or later turns to a specific affliction that has struck the lawmaker's family or friends. One look at the US Senate tells you that the National Institutes of Health can more or less write its own ticket when it comes to prostate cancer. Even a not very good Alzheimer's treatment is a sure bet for an enterprising drug company.

There's no doubt that medicine and smart environments can and will prolong life. Tools that are now imaginable - things like tailored viruses, cellular grafting, artificial glands, and artificial organs - could have a major effect; so could genotype choice. Medicine may not stretch out the baby boomers' lives much further than they can already expect (which is still longer than any other generation has been granted), but their children, or their children's children, could live a fair stretch longer, not least because it's probably going to be easier to preserve youth than to reverse decline. I'd be surprised if some of the younger readers of this article are not still in pretty good shape when it's time to see in the 22nd century, though I don't anticipate joining in the festivities myself. It's conceivable that if your children choose your grandchildren's genes well, some of them may see the 24th.

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It's a happy coincidence of iconography that the double helix was a medical icon long before the discovery of DNA. The two twisted snakes of the caduceus have been with doctors since Hippocrates, as the sacred symbol of Mercury, god of messages, of choices, of crossroads.

Now medicine can offer us more than ever before; more benefits and more choices. Better biology will not solve everything. Warding off many forms of cancer will not make the world perfect; nor will delaying the onset of Alzheimer's by a few decades, or even a century. Better understanding of the relief of emotional suffering will not make us all happy, and new techniques for maximizing the power of the intellect will not make everyone an Einstein. But the past five years suggest that all these things are conceivable over the next century - I'd say quite likely. We need to face them, decide how much we want them, look at ways we can spread their benefits widely, look at what they might cost us.

Nature has been making choices for 4 billion years; every base in the human genome is a result of those natural selections, just as every byte of a computer program is a choice between yes and no. This record has given us our knowledge, but it is no guide to how to use it. Those choices must be made not on the basis of what has worked before - which is all nature can ever offer us - but on the basis of what we want, individually and collectively. From now on the caduceus is ours.

So are the crossroads.