The US government should let scientists combine the DNA of three people to make an embryo, a panel organized by the US National Academies of Sciences, Engineering, and Medicine said in a report today. The procedure, which was approved by the UK House of Commons last year, is designed to help women with mitochondrial disease give birth to healthy babies. But the technique is controversial; if approved, it would mean that genetic components from three people would remain in a child for life.

What's perhaps even more controversial, however, is that the panel said the technique should only be used to create male embryos.

Each year, an estimated 4,000 people are born with mitochondrial disease in the US. The condition involves a genetic mutation that can cause a number of health problems, including seizures, blindness, heart problems, and liver disease. Because the disease is only passed down from mother to child, many women with the condition decide to adopt rather than run the risk of having an affected baby. But scientists thinks a gene therapy technique called mitochondrial replacement could let mothers have healthy children to which they're genetically related.

Here's how it works: DNA can either come from the cell's nucleus and or from mitochondria, which are tiny, energy-producing "batteries" located inside the cell. And unlike nuclear DNA, mitochondrial DNA is only passed down by the mother. To avoid passing on faulty DNA, scientists have suggested taking a donor egg from a woman with normal mitochondria, removing its nucleus, and replacing it with the healthy nucleus of an affected mother. Others think modifying a fertilized donor egg by transferring nuclear DNA from an affected mother might do the trick. But some regulators aren't sure the technique should be used at all.

Even though mitochondria only contribute 37 of a person's more than 25,000 genes, the social implications of having a child who technically has three parents are substantial. And the procedure, which has not yet used to produce a person, may not be safe. That's why the US Food and Drug Administration asked the panel to review the technique and make a recommendation.

In its report, the panel said the technique should be used in clinical trials — but only on male embryos. Because men don't pass down mitochondrial DNA to their children, this would stop modified mitochondrial DNA from being passed down through generations. This restriction differs substantially from last year's UK approval because lawmakers there included no restrictions on the sex of embryos. The panel also suggested a number of steps to ensure the safety of the procedure — steps that include keeping track of any children created with the procedure for years after they’re born. If the technique works in male embryos, it could then be used to make female children as well, the panel said.

The panel's conclusions are positive, Bruce Cohen, director of the NeuroDevelopmental Science Center and Pediatric Neurology at Akron Children’s Hospital, told the Genetic Expert News Service. And the fact that the panel wants to limit the use of the technique to male embryos is a good thing, he says; it avoids the question issue of what happens if the replacement doesn’t fully work or has pathogenic mitochondria, which means that it might quicken the process of developing therapies. "This is not in any way an attempt by the FDA or anyone else at gender selection and anyone who thinks that is misinformed," he says.

Eli Adashi, a medical science professor at Brown University, agrees. "The limitation of use of mitochondrial replacement to males was a surprising and clever twist on their part that makes a lot of sense," he told the Genetic Expert News Service. That said, if the FDA agrees with the panel, it would mean that the some of the fertilized embryos that are created won't be used simply because they're female — and "some may find [that] objectionable."