The smiling young woman in a red dress who stared out from the front page of The Times yesterday does not much look like the face of eugenics. Yet to her critics, she is about to cross an ethical boundary.

If the fertility watchdog grants permission to her doctor – and it has already agreed in principle – she will soon start IVF treatment, even though she is fertile. Her embryos will then be screened for a gene that heightens the risk of breast cancer. This gene killed her mother, her grandmother and her great-grandmother, and she carries it herself. Now she has an opportunity to spare her daughters this devastating heirloom.

For some, though, this is no cause for celebration. They fear instead that we are treading the path towards designer babies, created to feed pushy parents’ fantasies of raising the next Cristiano Ronaldo or Scarlett Johansson.

Their concern is that embryo screening, which uses a technique called pre-implantation genetic diagnosis (PGD), is being used for an ever-widening range of inherited conditions. It was originally licensed only for rogue genes that invariably cause an incurable and life-threatening disease, such as cystic fibrosis. Now, it has been deemed acceptable when the risk of illness is just 80 per cent, and for a cancer that can be prevented, albeit with mutilating surgery.

From here, the argument goes, it is a short jump to testing for DNA that is more loosely linked to disease, or to no disease at all. Doctors might screen for the recently discovered gene that raises the risk of obesity, even though diet and exercise play more important roles. Selection of socially desirable traits such as intelligence and good looks could follow. The parallels drawn are with the Nazis, Huxley’s hatcheries and the biological underclass of the science-fiction movie Gattaca.

It is easy, though, to get carried away by cliché. Talk of designer babies, slippery slopes and brave new worlds adds little to constructive debate about PGD, because its dystopian potential is firmly limited by science. It is certainly a powerful technology that has helped hundreds of families to have a healthy child. But it is entirely unsuitable for mass production of babies-to-order.

To begin with, PGD is impossible without IVF. To test an embryo’s genetic make-up, it is first necessary to grasp it with a pipette and extract a cell. This can be done only with an embryo fertilised in the laboratory. The procedure also requires a reasonable supply of embryos to test: if a single embryo conceived naturally is faulty, there would be no others to implant instead.

This necessarily knocks straight out of the game many couples who might like a designer baby. For all its undeniable benefits, IVF holds little appeal for the naturally fertile. Success rates are poor, and it involves invasive and gruelling gynaecological procedures. There are also side-effects for the prospective mother: menopausal symptoms such as hot flushes, and on rare occasions ovarian hyperstimulation syndrome, which can be fatal. Natural conception is a more efficient option for those who can manage it, not to mention more fun.

Then there is the matter of what to look for. At present, scientists know of about 200 single genes that cause or predispose to disorders for which it is reasonably simple to test. The kind of traits that parents would wish for in a true designer baby, however, are not much influenced by solitary genes.

Intelligence, height, good looks, athletic ability – all have a heritable element, but this is determined by a complex array of genes, with relationships that are poorly understood. Each trait will also be affected by environment, both in the womb and childhood: there is no guarantee that the designer baby will do what it says on the tin. It is one thing to expect PGD to detect cystic fibrosis, quite another to identify an Oxbridge first waiting to happen.

Some of these hurdles, of course, may eventually be overcome. Our understanding of how genes affect one another is progressing all the time, and it is not implausible that a genetic suite that enhances intelligence might eventually be pinned down. “Gene chips” that can test for thousands of genes at once are improving in sophistication.

But there is still the raw material to contend with. Embryologists performing PGD can work only with what nature provides. No matter how many embryos are produced, each will inherit all its genetic material equally from its parents. It is all very well ordering a designer baby with the brains of Stephen Hawking and the looks of Kate Moss. If mum and dad are thick and ugly, it isn’t going to happen. Create a thousand embryos if you like – you’ll be testing away in vain. PGD is incapable of making genetic haves of the offspring of have-nots.

Improved technology could actually make matters worse for would-be parents of a designer baby. In genetic screening, there is such a thing as too much information. Even when looking for a single, catastrophic mutation, such as the one that triggers Huntington’s disease, all the embryos that are created sometimes test positive. Add in the thousands of defects that a gene chip might one day reveal, and it will become nigh on impossible to find one that has nothing wrong with it. Do you select the embryo with “clever” genes but a propensity to schizophrenia? Or the athletic one likely to develop type 2 diabetes later in life?

Knowing about the natural limits to the utility of embryo screening is fundamental to informed debate about its place in medicine. To oppose embryo screening on the basis of implications that are largely imagined is to elevate implausible thought-experiment over proven potential to transform life and health. PGD is a great tool for preventing genetic diseases that cascade down the generations, blighting whole families with misery and suffering. For the eugenic design of superhumans, it is essentially useless.

Mark Henderson is science editor of The Times

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