Dani Garavelli - The gene genie

THE most awe-inspiring thing about newborn babies is that – to their parents at least – they are blank sheets of paper. OK, so their fathers may have them pegged as future Scotland strikers, or their mothers as candidates for the Bar. But when the midwife hands them over, they are really just bundles of potential waiting to be realised.

Part of the adventure of those early days is unravelling their mysteries: discovering if they are shy or extrovert; easygoing or high maintenance; bursting with health or physically fragile – and realising you will love them whatever.

This is true for most couples. But there are others – those at risk of having a child with a serious inherited condition – for whom the experience is terrifying rather than exhilarating. For them, pregnancy will be an agonising waiting game; and the birth may herald the start of a journey that, far from being a foray into the unknown, has already been mapped out as short and painful.

Last week's announcement that a test capable of screening IVF embryos for almost all of the 15,000 known genetic conditions (rather than the just 2% which can be identified through existing procedures) will soon be available has the potential to change such people's lives forever. Imagine: by selecting only the embryos which are free of the genetic mutation, they will be able to guarantee their baby does not inherit their condition, and save the NHS money into the bargain.

Like most technological advances then, the "genetic MOT" has clear benefits to a small percentage of the population. But, unfortunately, the sheer scale of the information that can be gathered by the process – known as karyomapping – makes it an ethical minefield for others.

Potentially, the test, developed at the Bridge Centre in London, could be used to screen out anything from the devastating Edwards syndrome (which claims the lives of 95% of sufferers in the womb) to haemophilia (which can be controlled). Since the technology is capable of identifying multiple genetic variations, it could also be used to test for combinations which carry a future risk of conditions such as diabetes, heart disease and cancer. It would even make it possible for parents to choose their baby's eye colour and to stack the odds in favour of it being a particular height or build.

The problem for both the medical profession and the layman is where to draw the line. Even if we, as a society, agree the screening process should be confined to serious genetic conditions, how are we going to define them? The furore over the abortion of a foetus with a cleft palate a few years ago demonstrates that one person's minor, rectifiable defect is a source of great anguish to another.

My particular concern is that, with society increasingly polarised along religious/secular lines, debate on this issue is being stifled. I find it hard to believe most people in the UK are so at ease with the concept of "designer babies" that they think there are no circumstances in which destroying embryos on the basis of a genetic flaw would be unacceptable. But, with non-believers unwilling to align themselves with the religious right, and lacking any other more moderate forum, their voices are going unheard.

Beyond the ethical questions are the practical ones. Even if you have no qualms about the morality of karyomapping, you would have to ask yourself what is to be gained from knowing your unborn baby's every genetic fallibility. Unless you are seriously suggesting you would dispose of an embryo on the basis it was predisposed to asthma, then what benefit would there be to knowing about the risk in advance? Indeed, couldn't it have a negative impact on your relationship with your child? The spectre of a condition they do not yet have might lead you to impose limitations on their activities.

And, as for finding out they had a risk of contracting a particular disease in 20 or 30 years, what purpose would it serve other than to cast a pall over an otherwise carefree existence? For all its promise, karyomapping can't offer a glimpse into the future. By the time the embryo in question gets cancer, there might be a cure. Then again, a couple might select an embryo without the cancer gene, only for the child it becomes to be killed in a road accident. Sometimes I think that in our pursuit of longevity we forget that we all have to die of something. Who is to say that ending our days in decrepitude is necessarily for the best?

Some of the fears raised by karyomapping are pie in the sky. For logistical reasons, it will still be impossible to guarantee a defect-free baby. "When you start looking for more than two or three traits, you've got no chance of getting a match," says Alan Thornhill, the scientific director of the Bridge Centre. "You'd need thousands of embryos, and we just don't have a practical way of making thousands of embryos."

Well, thank God for that. The thought of some reproductive factory spewing out conveyor belts full of them, just so one couple can conceive their ideal child, is just too Brave New World for comfort.

Even with the limits imposed, though, I don't think I would give my unborn child a genetic MOT. Not so much for moral reasons, but because I think there's something life-enhancing about learning to love the child you get. And because, far from being a liberating force, choice can bind us to our own narrow preconception of what perfection means.