Babies with three parents

Designer babies with three parents could be born within three years. The controversial technique screens an embryo created by a man and a woman for incurable genetic diseases. Defective DNA is replaced with that from another woman, effectively giving the baby two mothers and a father.

Scientists at Newcastle University in the UK have already created embryos using the method and are perfecting it for use in IVF clinics.They say it could free children from diseases including some forms of diabetes, blindness and heart problems. Critics say it could lead to genetically-modified babies being designed to order. UK law says embryos created using the technique must be destroyed, but scientists hope this can be overturned.

The research focuses on mitochondria "batteries" inside cells, which turn food into energy. Each mitochondrion has its own DNA, which is passed from mother to child. Defects in this DNA affect more than one in 5000 babies and cause around 50 genetic diseases, some of which kill before adulthood. The researchers have managed to swap the damaged DNA with healthy genetic material.

The first step is fertilisation of an egg through IVF. The embryo is screened for defects. When it is a few hours old, the nucleus containing genetic information from the parents is removed and put into another woman's healthy egg. Mitochondria are outside the nucleus so the baby is free of defects and will look like its "real" parents.

US biologist Professor Jonathan Van Blerkom says it would be "criminal" not to allow the technique to be used. There are fears that the influence of mitochondria on areas including longevity, IQ and fertility could lead to GM babies being made to order.