Nobody knows how many of them there are, but we know they exist. They are sometimes called "three-parent offspring," which sounds sinister. But if you met one, you would never know him from any other kid.
These children owe their existence to a fertility treatment called ooplasmic transfer, which involves taking an egg cell from an infertile woman and injecting it with the gel-like cytoplasm from another woman's egg. For some reason this can reverse infertility, possibly because it also transfers healthy mitochondria. Since these bits of cellular machinery contain DNA, children conceived this way inherit genes from their mother, father, and the donor, hence the "three-parent" tag.
The technique made headlines a decade ago when the U.S. Food and Drug Administration banned it. It is likely to get another airing as the world weighs up what to do about mitochondrial transfer—a form of IVF that can prevent genetic diseases by deliberately endowing embryos with donated mitochondria (see "Make three-parent babies, U.K. government told").
The evidence so far, from research in animals and very early human embryos, suggests that mitochondrial transfer works and is safe. Scientists who work on it believe it will soon be ready for human trials. There is certainly no doubt about its desirability—around 1 in 7,000 children are born with mitochondrial disease.
But technical progress only tells half the story. There are ethical issues, too. Mitochondrial transfer is a form of "germ-line" genetic engineering, which is generally seen as unacceptably risky because it doesn't just alter the DNA of the child who receives it but also that of future generations. For that reason we do not engineer out diseases like cystic fibrosis even though it is technically feasible to do so.
Mitochondrial engineering is somewhat different, though. The mitochondrial genome is tiny, so the changes involved are minimal. The public seems to understand this: A consultation in the United Kingdom recently found that most people think the benefits outweigh the risks of changing the germ line.
In any case, mitochondrial transfer does not necessarily entail germ-line engineering. Mitochondria are inherited exclusively down the maternal line, so only female children could pass on donated mitochondria to their own children. That means there is the potential to limit the engineering to a single generation by only offering it for male embryos.
Of course, gender selection for medical purposes raises issues of its own, but not major ones. In some countries it is already routinely offered to couples who have a high risk of passing genetic diseases to their sons.
It has also been used in at least one case of mitochondrial disease. Last year an American woman who is a carrier—which means she is healthy but at risk of passing the disease to her children—used standard genetic screening to select an embryo with low levels of faulty mitochondria. She also chose a male embryo.
So what about the "three-parent" issue? It is of questionable relevance. Mitochondria contain genes but make no contribution to the traits that make us human—personality, intelligence, appearance, and so on. A donor would have no more right to claim parenthood than someone who gives blood. In any case, the precedent set by ooplasmic transfer suggests that the issue has no practical significance.
It may be that, in the end, mitochondrial transfer fails to get through human trials. But the ethical concerns need not be a barrier to trying. The potential dividends for human well-being are too great.
This editorial originally appeared in New Scientist.