Cystic fibrosis, Duchenne muscular dystrophy and Huntington’s disease are among the most terrible illnesses that a human being can endure. All are caused by a genetic defect. Victims are usually struck down early and lead short, painful lives. Almost invariably, there is no cure. The discovery, therefore, of a successful technique to screen embryos to identify these diseases is one of the clear miracles of bio-engineering that has brought huge relief to worried parents and could, eventually, prevent a vast amount of suffering.

The technique, called pre-implantation genetic diagnosis (PGD), consists of removing a cell from an embryo, testing it for any genetic abnormality then implanting the embryo in the womb — provided that it is clear of the defects that could trigger these diseases. Permitted under strict guidelines laid down by the Human Fertilisation and Embyrology Authority, PGD is, in Britain, allowed only for a narrow range of conditions, and its use must, in each instance, have formal medical sanction. For the technique is still controversial. It is not just the ethical objections to discarding an embryo found to be damaged; the technique raises the whole issue of “designer babies” and could be the slippery slope that leads to the abuse of sex selection or the birth of babies with chosen genetic characteristics.

Those fears may now be heightened by the landmark ruling to allow PGD to be used for patients with a genetic form of bowel cancer and by the likely application to extend the procedure to breast cancer. These diseases, though causing widespread suffering to many people, fall into a different category. Both can be caused by faulty DNA, but not all those inheriting the gene fall ill. Some of those who do can be cured; others can opt for pre-emptive mastectomy. The onset is generally later in life, allowing many patients to enjoy years of normal health.

This is even more the case with Alzheimer’s, which does not occur until late in life. Will parents really want to deny their child life because an embryo tested positive for a genetic trigger? The numbers involved will be small, as few women would opt for a technique as invasive and expensive as PGD without good reason.

In theory, strict regulation means that decisions are taken only after debate informed by the expertise of doctors. But that very expertise may change the debate again by finding cures for debilitating disease. In Britain, the technique is unlikely to be misused. In other countries this may not be so. In India or China, for example, the societal pressures to produce male babies could be so overwhelming that a doctor could connive in selecting embryos according to sex rather than screening for genetic defects.

There will always be the difficulty of where to draw the line. Which disease is serious enough for parents to screen? What about a history of mental illness? Parents have a profound responsibility, as does the medical profession. PGD has the potential to prevent incalculable suffering; it must be very selectively used, not abused.