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MELISSA BLOCK, HOST:

This is ALL THINGS CONSIDERED from NPR News. I'm Melissa Block.

AUDIE CORNISH, HOST:

And I'm Audie Cornish. Scientists have crossed a controversial threshold. They have made changes in human DNA that can be passed down through generations. As NPR's Rob Stein reports, the aim is to prevent genetic diseases in babies, but the work is raising a lot of ethical questions.

ROB STEIN, BYLINE: Scientists have been tinkering with human genes for decades, but Marcy Darnovsky of the Center for Genetics and Society says there's always been one big no-no: don't make changes that would become a permanent part of the human genetic code.

DR. MARCY DARNOVSKY: That kind of genetic engineering has been ruled off-limits. And it's a very bright line that has been observed by scientists around the world.

STEIN: There have been lots of reasons for this. One big one is purely practical, says Dartmouth bioethicist Ronald Green.

DR. RONALD GREEN: If we make mistakes, we'll effectively be introducing a new genetic disease into the human population for generation after generation.

STEIN: But Green says taking that step has raised more far-reaching fears. It's the kind of thing that could potentially be used to create genetically superior humans.

GREEN: It could easily move into the realm of gene enhancement, higher IQ, improved physical appearance, athletic ability. That's a worry to some people, to many people.

STEIN: But in this week's issue of the journal Nature, researchers report they have crossed that line. They have figured out how to change the DNA in a human egg. Mary Herbert of Newcastle University in England says the goal is to prevent rare but horrible genetic disorders in babies, disorders caused by defects in DNA that only mothers pass down to their kids.

MARY HERBERT: In severe cases, the child will die in the first days of life, or they might live, you know, a few years and then die. It's like a game of Russian roulette. Very difficult, very difficult reproductive choices.

STEIN: So a team of scientists at the Oregon Health & Science University figured out how to pluck the defective DNA out of a woman's egg and replace the bad genes with healthy DNA from eggs from other women. Shoukhrat Mitalipov led the team. He says the geneticists fertilize the transplanted eggs in the lab and showed they could create healthy embryos.

DR. SHOUKHRAT MITALIPOV: We showed that the faulty genes, which are usually passed through a woman's egg, can be safely replaced. And that way, the egg still retains its capacity to be fertilized and developed.

STEIN: The Oregon team hasn't tried the next step yet. They haven't tried to make babies out of these modified embryos. But they have made healthy baby monkeys this way, making them confident it could work. Mary Herbert's team in Newcastle has sparked a national debate in England by doing similar research.

HERBERT: If it is within our power to relieve suffering, then, to my way of thinking, there is an ethical onus on us to do that.

STEIN: But other experts aren't so sure. They question the morality of creating human embryos in the laboratory only to sometimes destroy them. Others wonder how scientists will ever know whether this is safe enough to use to try to create a baby. And there's more. There's the genetic identity of babies made this way. Ronald Green at Dartmouth says they'd inherit DNA from the father, from the woman whose egg was defective and from the woman who donated DNA to fix the problem.

GREEN: So, yes, we're going to have to perhaps get used to the fact that people can have three genetic parents in the future.

STEIN: But beyond that, the move raises those fears about manipulating human DNA, of using these techniques to create some kind of brave new world of genetic haves and have-nots, according to Marcy Darnovsky.

DARNOVSKY: Socially, what this would mean is we would be moving toward a world in which some people, and it would be people who could afford these procedures, would have either real or perceived genetic advantages.

STEIN: Despite these concerns, the scientists in Britain and Oregon say the real benefits of preventing genetic diseases outweigh such hypothetical risks. A British bioethics group recently concluded the research is ethical as long as it's proven to be safe. Herbert is awaiting a decision by the British government about whether she can take the next step in her experiments. The Oregon team is waiting for the Food and Drug Administration to rule on whether they can try to make healthy babies by genetically altering human eggs. Rob Stein, NPR News.

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