Roughly one baby in 6,500 is born with misfiring mitochondria, the tiny power plants found in virtually every cell that release energy from food and oxygen. That can cause a long list of problems, all of which are unpleasant, and many of which are fatal. They include diabetes, deafness, debilitating muscle weakness and progressive blindness, as well as epilepsy, liver failure and dementia. Some afflicted babies die shortly after birth. Others face a life of permanent ill-health. At the moment, such diseases are simply a tragedy that must be lived with. But doctors in Britain and America are working on a cure. If they can perfect a new technique, and if they can persuade the world's governments to legalise it, it will mark a significant moment in medical history, and not just for the benefits it will bring. For one thing, babies born via this technique would possess DNA from three people—the mother, the father and an unrelated egg donor—rather than the usual two. And it would be the first time that a genetic treatment has been licensed that affects not just the individual in question, but his or her descendants, too. How does it work?
The treatment relies on the fact that mitochondria are not just another part of a living cell. They are the distant descendants of bacteria that, a billion years ago, gave up their free-living lifestyle to form symbiotic partnerships with other cells. As a result, mitochondria possess their own tiny genomes, entirely separate from the much bigger hunk of DNA that sits inside the cell nucleus. A baby inherits its "nuclear DNA" almost equally from its mother and father. But it inherits its mitochondria only from its mother: every single one is a descendant of the mitochondria from the mother’s egg cell. Although the British and American researchers are using different techniques, the basic idea is the same: to give the baby a fully working set of mitochondria donated by another woman. The scientists take an egg with damaged mitochondria, remove the nucleus (and the DNA it contains) and transplant it into a second, donor egg, whose nucleus has been removed but whose mitochondria are working normally. The result is a baby that will have nuclear DNA inherited from its mother and father in the usual way, but mitochondria inherited from the egg donor.
