Scientists in Oregon have created embryos with genes from one man and two women, using a provocative technique that could someday be used to prevent babies from inheriting certain rare incurable diseases.

The researchers at Oregon Health & Sciences University, in a report published online Wednesday by the journal Nature, said they are not using the embryos to produce children, and it is not clear when or even if this technique will be put to use. But it has stirred a debate over its risks and ethics in Britain, where scientists did similar work a few years ago.

The British government is asking for public comment on the technology before it decides whether to allow its use. One concern it cites is whether such DNA alteration could be an early step down a slippery slope toward "designer babies."

'Might not be the best way'

Questions have also arisen about the safety of the technique, not only for the baby who results from the egg, but also for descendants. In June, a British bioethics group concluded that the technology would be ethical to use if proven safe and effective.

Laurie Zoloth, a bioethicist at Northwestern University, cautioned that safety problems might not show up for several generations. While the kind of diseases it seeks to fight can be terrible, "this might not be the best way to address it," Zoloth said.

Over the past few years, scientists have reported that such experiments produced healthy monkeys and that tests in human eggs showed encouraging results. The Oregon scientists reported that they have produced about a dozen early human embryos and found the technique is highly effective in replacing DNA.

The genes they want to replace aren't the kind most people think of, which are found in the nucleus of cells and influence traits such as eye color and height. Rather, these genes reside outside the nucleus in energy-producing structures called mitochondria. These genes are passed along only by mothers, not fathers.

How it would work

The new technique, if approved someday, would allow a woman to give birth to a baby who inherits her nucleus DNA but not her mitochondrial DNA. Here's how it would work: Doctors would need unfertilized eggs from the patient and a donor. They would remove the nucleus DNA from the donor eggs and replace it with nucleus DNA from the patient's eggs. So, they would end up with eggs that have the prospective mother's nucleus DNA, but the donor's healthy mitochondrial DNA.

The procedure is a way of replacing some defective genes that sabotage the normal workings of cells; the donor DNA amounts to less than 1 percent of the embryo's genes

About 1 in every 5,000 children inherits a disease caused by defective mitochondrial genes. The defects can cause many rare diseases with a host of symptoms, including strokes, epilepsy, dementia, blindness, deafness, kidney failure and heart disease.

Lead researcher Shoukhrat Mitalipov said he hopes to get federal approval to test the procedure in women.