Can a baby have two mothers?

Not really. Reports this week that scientists plan to create children with two mothers are stretching a point.

"A gross exaggeration," says Howard Cooke, at the MRC Human Genetics Unit in Edinburgh. "Ambiguous," says Joanna Poulton, professor of mitochondrial genetics at Oxford.

The story was sparked by a team of scientists at Newcastle University applying to the Human Fertility and Embryology Authority to study a range of diseases that are often fatal in childhood. They are caused by mutations in DNA wrapped up in tiny structures called mitochondria, which live in the cell body but not the nucleus and act as microscopic engines, turning sugars into fuel.

Unlike the genes in a cell's nucleus, mitochondrial DNA has a negligible effect on our appearance. But if the DNA is damaged, it can disrupt our ability to make energy, leading to conditions as familiar as diabetes, or the more rare Leigh's disease, which causes dementia and is often fatal before children reach their teens.

The Newcastle group hopes such diseases might be cured by extracting the nucleus from an affected egg and implanting it into another egg that has had its nucleus removed, but still contains healthy mitochondria.

If the research were ever to become a therapy, the resulting children would have DNA from both parents, plus healthy mitochondrial DNA from an egg donor. Does that amount to two mothers and one father? "We have thousands of genes on our chromosomes but only tens in our mitochondria, so I would describe them as babies with 2.001 parents," says Cooke, adding: "You'd never know what was due to mitochondria and what was down to the environment."