An influential science advisory group formed by the National Academy of Sciences and the National Academy of Medicine on Tuesday lent its support to a once-unthinkable proposition: the modification of human embryos to create genetic traits that can be passed down to future generations.
This type of human gene editing has long been seen as an ethical minefield. Researchers fear that the techniques used to prevent genetic diseases might also be used to enhance intelligence, for example, or to create people physically suited to particular tasks, like serving as soldiers.
The advisory group endorsed only alterations designed to prevent babies from acquiring genes known to cause ''serious diseases and disability,'' and only when there is no ''reasonable alternative.'' The report provides an explicit rationale for genetic research that the federal government has avoided supporting until now, although the work is being pursued in countries like Sweden and China.
So-called germ line engineering might allow people to have biological children without fear that they have passed on the genes for diseases like Huntington's, Tay-Sachs and beta thalassemia, and without discarding embryos carrying the disease-causing mutations, as is often done now. Though such cases are likely to be rare, the report says they should be taken seriously.
The new report heralds a day scientists have long warned is coming. After decades of science-fiction movies, cocktail party chatter and college seminars in which people have idly debated the ethics of humanity intervening in its own evolution, advancing technology dictates that the public now make some hard choices.
''It is essential for public discussions to precede any decisions about whether or how to pursue clinical trials of such applications,'' said R. Alta Charo, a bioethicist at the University of Wisconsin-Madison and a leader of the panel that wrote the report. ''And we need to have them now.''
Just over a year ago, an international group of scientists said it would be ''irresponsible to proceed'' with making heritable changes to the human genome until risks could be better assessed and there was ''broad societal consensus about the appropriateness'' of any proposed change.
No one is pretending that such a consensus now exists. But in the year that the committee was deliberating, Ms. Charo said, the techniques required to perform this sort of gene editing have passed crucial milestones.
The advent of a powerful gene-editing tool called Crispr-Cas9 allows researchers to snip, insert and delete genetic material with increasing precision. It has led to plans for experimental treatments of adult patients with cancer, blindness and other conditions as early as this year.
But these types of genetic alterations are not inherited.
''Previously, it was easy for people to say, 'This isn't possible, so we don't have to think about it much,''' said Richard Hynes, a cancer researcher at the Massachusetts Institute of Technology, who helped lead the committee with Ms. Charo.
''Now we can see a path whereby we might be able to do it, so we have to think about how to make sure it's used only for the right things and not for the wrong things,'' he said.
A more pragmatic concern driving the committee was the likelihood that the new technology would be adopted in countries like China, where some pioneering research on editing human embryos -- without the intent to gestate them -- has already occurred.
''If we have an absolute prohibition in the United States with this technology advancing, it's not like it won't happen,'' Ms. Charo said.
But opponents of human germ line editing say that is not a reason to take a big step toward what they fear will be an inevitable push to engineer traits like strength, beauty and intelligence, perhaps eventually creating a dystopian social divide between those who can afford enhancements and those who cannot.
''This opens the door to advertisements from fertility clinics of giving your child the best start in life with a gene-editing packet,'' said Marcy Darnovsky, the executive director of the Center for Genetics and Society, a public interest group in Berkeley, Calif. ''And whether these are real advantages or perceived advantages, they would accrue disproportionately to people who are already advantaged.''
The new guidelines, Ms. Darnovsky noted, also set the United States apart from many European countries that have signed a treaty to refrain from human germ line editing.
In addition to social concerns, there are questions of safety and autonomy. While Crispr is generally precise, it can have ''off-target'' effects, cutting DNA at places where it is not meant to. What if a child produced through a gene-editing technique were hobbled in some unforeseen way?
The new report called for prohibiting any alterations resembling ''enhancement'', including ''off label'' applications. Under the guidelines, a genetic technique aimed at strengthening the muscles of patients with Duchenne muscular dystrophy, for instance, could not be used to make healthy people stronger.
But it is not clear who would draw those lines.
George Church, a geneticist at Harvard University, saw no clear prohibitions against certain enhancements in the report. ''If these fixes for severe diseases are shown to be safe and effective, why would small or large enhancements accompanying the fixes be unacceptable?'' he asked.
No one should expect to design a baby anytime soon. It will probably be years before gene-editing techniques tested in animals can be shown to work in humans. And for the moment, the Food and Drug Administration is prohibited from using federal money to support research that results in genetically modified offspring.
In the meantime, said Sharon Terry, the president of the Genetic Alliance, a patient advocacy group, patients who may participate in clinical trials must begin a conversation. ''My hope is that there would be serious considerations about what we are balancing here,'' she said.
Follow Amy Harmon on Twitter: @amy_harmon
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PHOTO: Embryos being removed from cryogenic storage. Scientists have taken a step forward in what has long been an ethical minefield. (PHOTOGRAPH BY UNIVERSAL IMAGES GROUP, VIA GETTY IMAGES)