A US couple have used genetic testing to ensure that their second child, a boy, is not only free of a rare inherited disease, but also able to provide his ill sister with a vital blood cell transplant. Jack and Lisa Nash used preimplantation genetic diagnosis (PGD) - a technique in which IVF embryos can be tested for an inherited trait before implantation - to select a healthy embryo with a tissue type matching that of their six-year old daughter Molly.

Molly Nash has Fanconi's anaemia, a life-threatening condition with a range of symptoms, including a lack of healthy blood stem cells in the bone marrow. Mr and Mrs Nash wanted more children, but knew they had a 25 per cent chance of conceiving another affected child. So the couple underwent four cycles of IVF, the last of which resulted in fifteen embryos. Using PGD, doctors found that two were free of the disease and also potentially able to provide a matched transplant for Molly. One was healthy enough to transfer to Mrs Nash's womb, and Adam Nash was born on August 29. A month later, blood stem cells from his umbilical cord were transplanted into Molly, who now has an 85 to 90 per cent chance of making a full recovery.

The case has attracted worldwide media interest, with some commentators expressing concern that the achievement could lead to 'designer babies' selected on the basis of non-disease traits such as athletic ability. But Dr Paul Veys, a bone marrow transplant expert at Great Ormond Street Hospital, London, says the Nash's decision was ethically justified. 'In this case, hopefully you're going to end up with two children who are healthy' he said. 'How can you argue against that?'