All about the boys Ultra.F/Getty

Women in the US whose children risk inheriting severe mitochondrial diseases should be allowed to have the mitochondria in their eggs replaced by those from a healthy donor, says the Institute of Medicine. The caveat? They should only be allowed to have sons. This will ensure their grandchildren will not be affected if there turn out to be any unforeseen consequences with this controversial form of IVF.

A growing number of diseases – including some that cause severe disability or death in childhood – are linked to genetic mutations in mitochondria, the energy-producing organelles in our cells that have a tiny genome of their own. Replacing the faulty mitochondria in an egg or embryo with those from a healthy donor should prevent these diseases.

The Institute of Medicine’s report was requested by the Food and Drug Administration, so its recommendations are likely to become the official policy of the US regulator.


The sons-only recommendation has been criticised by researchers in the UK, whose parliament gave the go-ahead for mitochondrial replacement therapy last year without any such restriction.

The next generation

“I think the UK came to the right conclusion,” says Alison Murdoch of Newcastle University in the UK, whose team is developing mitochondrial replacement therapies. “This would rather defeat the purpose of what we are trying to do.”

The technique is a form of genetic modification because a small part of the DNA of children created this way would come from the donor. Because mitochondria are inherited from the mother, if a women has a daughter, this would be passed down to the next generation. If she has a son, the donor mitochondria gets passed to him but no further.

Up to 17 people in the US may already have been born with donor mitochondria, because of a technique one clinic used to boost the success of IVF between 1997 and 2002 – when the FDA stepped in to stop it.

If the FDA follows the Institute’s recommendations it will allow donor mitochondria to be used to prevent children inheriting serious diseases – but the sex of the embryos should be determined, and only male embryos transferred to women. This is despite it acknowledging that there is a debate about whether sex selection is ethically acceptable.

Stressed embryos

This approach was carefully considered when the UK was formulating its policy, says Douglas Turnbull of the Wellcome Trust Centre for Mitochondrial Research in the UK. But there are three main reasons it was rejected.

First, it reduces by at least half the chances of a women having a successful pregnancy, Turnbull says, because none of the female embryos can be used.

Determining the sex of an embryo involves removing a single cell to test – which could lower the chances of success by adding to the stress on embryos that have already undergone a fair amount of manipulation to replace the mitochondria. “You never want to do anything to an embryo if you don’t have to,” says Murdoch.

It will also be about 20 or 30 years before any of the children created with donor mitochondria are ready to have children of their own, says Turnbull, by which time there will have been tremendous advances. “Goodness knows what will happen in 30 years in technology.”