The Big Question: Should we be worried by the latest step towards designer babies?

What is the fuss about?

British scientists yesterday announced a breakthrough in genetic testing which will allow embryos to be screened for almost any inherited disorder passed on from their parents.

The announcement by specialists from Guys and St Thomas' Hospital was made at the conference of the European Society of Human Reproduction and Embryology. Five British women are pregnant having been screened using the new technique. The announcement provoked an immediate outcry, with the scientists claiming the advance represented a "paradigm shift" in the screening of defective genes, while critics claimed it was another step towards eugenics - the selection and disposal of "imperfect" humans.

What will the new development make possible?

About 200 inherited defects, including Huntington's disease and the commonest form of cystic fibrosis, can be identified in embryos by pre-implantation genetic diagnosis (PGD). However, the technique is limited to specific diseases because it relies on taking a single cell and testing it for the minute quantities of DNA that indicate a known mutation.

The new technique, known as pre-implantation genetic haplotyping (PGH), multiplies the entire genetic complement of the single cell a millionfold, allowing thousands of heritable conditions to be detected without the need to know the precise details of the genetic mutation.

Why is this controversial?

By selecting out, and disposing of, embryos affected by these disorders, the technique brings a step nearer the prospect that they might be eliminated altogether. Many doctors would welcome this, arguing that this is a case where medical technology is being harnessed to improve on nature, not distort it. But disability groups say it would increase discrimination. The British Council of Disabled People said: "It would create a society where only perfection is valued."

Have church leaders taken a view?

Yes. Pope Benedict XVI delivered a fierce condemnation of scientists who meddled with the genetics of babies last Easter. He described attempts to clone a baby with no father as an attack on the family, adding: "There is a move to re-invent mankind, to modify the very grammar of life as planned and willed by God." The Archbishop of Canterbury, Rowan Williams, has expressed concern about abortion.

Will such selection become widespread?

No. It will only ever be available to couples undergoing IVF - a lengthy, stressful and expensive procedure that is a last resort for those who are unable to reproduce naturally or have an inherited illness in the family that they are anxious not to pass on. The new PGH test announced yesterday costs £4,100 a time, which would be added to the costs of IVF. There are about 25,000 babies born annually in UK by IVF, compared with over 600,000 naturally.

Are we on a slippery slope?

The limits of what is acceptable are constantly shifting. At the moment, PGD is used for couples with a family history of a particular disorder. These are conditions, such as cystic fibrosis and Huntington's disease, which affect people in childhood.

Last month, the Human Fertilisation and Embryology Authority announced an extension of the range of conditions PGD could be used to test for, to include the genes linked with breast and ovarian cancer. Women with the genes have an 80 per cent chance of developing breast cancer - usually later rather than earlier in life. There is only a 40 per cent chance of developing ovarian cancer. Both are treatable. Women with these genes may therefore lead the whole or a large part of their lives free of disease. This has raised doubts about which embryos it is legitimate to "select out".

Has the breakthrough accentuated this dilemma?

Yes. Some diseases, such as Duchenne's muscular dystrophy and haemophilia, only affect boys. At present, screening for these disorders involves selecting only female embryos. The new technique will allow male embryos to be checked for the mutant genes and "healthy" male embryos replaced in the mother's womb. This means that parents with Duchenne's or haemophilia will sometimes be in the position of having healthy male and female embryos to choose from - and could select a boy or girl for social rather than medical reasons. Scientists say these cases will be rare.

Are parents allowed to choose the sex of their offspring?

No. The HFEA bans selection of embryos for "social, physical or psychological" reasons, to allay fears over designer babies being selected for IQ, athletic prowess, or hair colour. But where doctors and parents need to know the sex of an embryo for medical reasons, as in Duchenne's and haemophilia, and have a selection of equally viable male and female and embryos from which a choice has to be made, there is nothing to prevent them doing so.

What about the elimination of inherited diseases?

Some people carry defective genes, such as that for haemophilia, which only appear in their children (or later generations), leaving them unaffected. The new technique will allow carriers to be detected at the embryo stage and selected out. This will remove the risk to subsequent generations - but also deny life to an individual who would not themselves have been affected. The doctors at Guy's and St Thomas' say they do not intend to use the technique this way - but it represents a new ethical frontier.

Scientists talk about selecting out affected embryos - but what about selecting them in?

Some families, struck by rare conditions, have sought to use genetic testing to produce "saviour siblings" by ensuring they are a match for older children, whose only hope is a bone marrow transplant. The method has been approved by the HFEA, though critics have condemned it as "baby farming". The HFEA says it frames its policies to reflect changing public attitudes. A recent consultation revealed that 80 per cent of the public were opposed to sex selection for social reasons. The Government is planning to update the 1990 Human Fertilisation and Embryology Act.

Is more genetic screening of embryos to be welcomed?

Yes...

* Modern medicine is being harnessed to allow couples to plan healthy families.

* The techniques are being used to avoid unnecessary suffering.

* There are strict controls preventing selection of embryos for social, physical or psychological reasons.

No...

* This is the misuse of medicine, not to treat and cure, but to diagnose and eliminate the imperfect.

* It will increase intolerance of, and discrimination towards, people with disabilities.

* Scientists should stop playing God and trying to "re-invent mankind".