THE three-parent baby – created from the DNA of a mother and a father, plus another woman's egg– is soon to be part of our world.

Though it might sound like science fiction, the technology to create such an infant was approved last week by the Human Fertilisation and Embryology Authority (HFEA).

Consultation revealed there is public support for the technology that will prevent the passing from mother to child of a genetic condition called mitochondrial disease, which causes much suffering, including blindness, muscle weakness, liver, heart and gut disorders, learning disabilities and early death. The HFEA approved it because there are women out there who fear having a baby because they know this occurs in their family and that they are carriers.

Mitochondrial replacement involves fusing the egg-cell nucleus of the affected mother with an egg cell from a donor who doesn't carry the problematic gene. Effectively, it means putting the nuclear DNA of one woman into the egg of another. Given the suffering the procedure may prevent, who could object? Well, I might. This represents a new development in genetic selection against disease. It is a breakthrough that leads us that bit further towards a world in which we can control our genetic destinies.

My qualms are not so much about this technology, but about the ground that it stands on: a slope that includes breakthroughs such as the creation of the first genetically engineered human cell at Cornell University in the United States five years ago, and the HFEA's approval of pre-implantation screening for certain breast cancer genes. It is a slope that leads, slowly but surely, towards the designer baby.

Not everyone agrees this slippery slope exists. Scientist Stephen Pinker has described the designer baby notion as a myth, one which he believes is too difficult in terms of science to create, too risky, and too far off to be worth considering a problem.

But Pinker may be wrong. Around a decade ago he was arguing that designer babies were unlikely, given that the human genome was so complex and we had "not even yet found a gene for schizophrenia". Then in 2006 it was announced that Edinburgh University researchers had found a gene linked to the development of psychotic symptoms.

In the last month there has been news that autism, attention deficit hyperactivity disorder (ADHD), schizophrenia and bipolar disorder may be linkable to five mutations. We may not be very close to achieving the designer baby, but we are close enough to need to consider how we navigate this territory ethically.

Many object to such technologies on the grounds of safety, or because they believe there is something divine in the workings of the genetic lottery that produces a human being. But my fear is that we are on a journey that will lead to a loss of human diversity – what I fear is not super-enhancement, but super-normality.

Most people see genetic selection against disease as a separate issue from genetic enhancement and the designing of the super-baby. But philosopher Peter Singer says: "There is no bright line between selection against disabilities and selection for positive characteristics. From selection against Huntington's disease it is no great step to selecting against genes that carry a significantly elevated risk of breast or colon cancer, and from there it is easy to move to giving one's child a better than average genetic health profile."

There are difficult questions to ask about what we get rid of. James Watson, the scientist who with Francis Crick discovered the structure of DNA, once said: "If you really are stupid, I would call that a disease. The lower 10% who really have difficulty, even in elementary school, what's the cause of it? A lot of people would like to say, 'Well, poverty, things like that.' It probably isn't. So I'd like to get rid of that, to help the lower 10%." Getting rid of stupidity sounds like a good thing. But why stop at the lower 10%? Where exactly do we stop?

In some ways, gene technologies will only make more pertinent issues that are already there in our society. The debate over what is a disability and what is a "culture", what is to be cured and what lived with, has long raged. Deaf parents have argued for their right to have deaf children. In 1994, New York Times writer Andrew Solomon reported on a campaign by deaf people against a cure for deafness. He noted: "Perhaps it would be most accurate to say that 'disability' and 'culture' are really matters of degree. Being deaf is a disability and a culture in modern America; so is being gay; so is being black; so is being female."

But, just as we have almost eliminated certain infectious diseases, surely some genetic diseases are worth banishing from the gene pool. It seems fairly clear that something like mitochondrial disease is only going to cause suffering, and should rightly be prevented. If I were a woman carrying the gene, I imagine I would choose not to pass those genes on to a child. Does that mean I might also consider genetic technologies for lesser conditions? Like many, I balk at the idea. Though not religious, I tend, in this one area of life, to attach romantic significance to what the roll of the dice delivers.

This could easily be dismissed as superstition or nonsense. But is it? One of the commentators who has looked long and hard at what might be behind these feelings is the philosopher Michael Sandel, who in his book on genetic engineering, The Cost Of Perfection, wrote of how these technologies create a sense of "hyperagency" that undermines our sense of life as a "gift", given to us and not entirely in our control.

We need to hold on to this idea. As we begin to run down the slope of genetic technology, we need to remember the simple notion of life as a gift, whatever way it comes.