National news coverage of the F.D.A. meeting tended to frame the risk-benefit analysis in an even broader way: “You’re starting off with a technique meant to prevent devastating illness,” Jon LaPook, chief medical correspondent of the “CBS Evening News With Scott Pelley,” said, “but there are some people who worry that down the road, it could be used to try to make so-called designer babies, kids who are more intelligent, who have other qualities that the parents find desirable.” A 2009 report by Richard Harris, a science correspondent for NPR, pushed this idea further: “It could open the door to genetically engineering a lineage of people with supposedly superior qualities. This is called eugenics, and many people find that repugnant.” Biologically speaking, however, mitochondrial replacement cannot guarantee any traits, superior or otherwise, except, if it works as planned, the absence of mitochondrial disease. In a way, the procedure would cross the germ line on a technicality: It would replace genetic material, but it wouldn’t “modify” or “engineer” genes in the same way that bacterial DNA is added to a corn gene, for instance, to create a pest-resistant crop. So what, exactly, are we so afraid of?

We know the double helix as our identity; we take personally the thought of tampering with it. One of the first times researchers tried to discuss altering DNA — specifically, whether or not to splice together the DNA from different organisms, such as bacteria, to create new life-forms — at a conference in Asilomar, Calif., in 1975, the result was a public-relations disaster. “The scientists emphasized the awesome and mysterious technology and in doing so made it un-understandable and alien to the population at large,” Willard Gaylin, co- founder of the Hastings Center, a nonpartisan bioethics research group, wrote in The New England Journal of Medicine in 1977. “Asilomar became a scientific version of ‘Jaws,’ and the public was titillated but also frightened.” Gaylin called that fear the Frankenstein factor and warned that it would unconsciously “move public opinion, even though that fear will be rationalized by overt use of more realistic arguments.” (In the end, the researchers did splice DNA; this has lead to many benefits, but not, as feared, a cancer-causing superbug.)

Consenting to the crossing of the germ line would be another watershed, and the F.D.A.’s open-door meeting in February held the promise of revealing if the public was ready for the moment. The stated purpose of the event was not to render any decisions but to “inform potential future regulatory deliberations and actions.” Victory for the two U.S. scientists who have been pioneering the new technique and were presenting their work at the hearing would mean persuading the American public, not just the F.D.A., that clinical trials are both scientifically and ethically warranted. In Britain two years ago, the government asked for seminars to be held in which randomly selected citizens learned the nuances of how mitochondrial replacement works and then discussed the philosophical issues involved. Most of the participants ended up “broadly in favor” of it if proved safe. In the United States, it seemed as if the most vocal members of the public felt disturbed by the technique without necessarily being able to articulate why: Of the nearly 250 emails the F.D.A. received before its February meeting, most objecting to “three-parent babies,” more than half of them were form letters.

One problem the U.S. scientists had been having selling mitochondrial replacement was explaining what it was and what it would and wouldn’t be good for. Though no evidence indicates the technique can treat infertility, it might. That the early cytoplasm injections worked for some women (for unknown reasons) and involved the transfer of mitochondria has reinforced this idea. Also, as we get older, the mitochondria in all our cells become less efficient at generating power, and Shoukhrat Mitalipov, one of the F.D.A. presenters, from Oregon Health and Science University, has theorized that replacing the mitochondria in the eggs of infertile older women with donor mitochondria might rejuvenate them. “I believe that rationale is unfounded,” Mary Herbert, another presenter, from Newcastle University, told me. “I worry that it will give older women who want to conceive false hope.” What’s more, some who support mitochondrial replacement for women with mitochondrial disease are made nervous by the idea of using it to treat infertility, which is far more common, out of a sense that sheer demand could unleash it before its dangers are fully known.

Stories like that of Maureen Ott, whose daughter, Emma, now 17, gets straight A’s, is senior-class treasurer and plays varsity sports, can’t help shaping the debate. Sharon Saarinen had the injection in Michigan in 2000, when she was 36. “I don’t remember them mentioning any risks,” she says. “If there were risks, it didn’t matter. I wanted a child too much at that point.” Her daughter, Alana, like Emma, is exceptionally bright and healthy; she has never been tested to see if she has the DNA of three people — she might or might not. “From Day 1, I’ve always felt this was a miracle procedure for me,” Saarinen says. “As my daughter grew and she’s fine and so intelligent, it just backed my belief that it was the right thing to do.” These cases aren’t statistically significant; they prove nothing about the safety or effectiveness of cytoplasm injection or having a third person’s mtDNA. Medically speaking, they are barely relevant to the new technique at all. But for better or worse, they are part of the collective narrative about crossing the germ line and thus have an outsize effect on our view of it. Americans were uneasy with I.V.F. and its “test-tubes” until they saw that it created unblemished babies; 40 years and millions of births later, we now accept it as routine. It’s easy to imagine the same chain of events taking place with mitochondrial replacement and turning out just as happily — or, several decades in, discovering an unexpected problem.

Three days before the F.D.A. hearing, I called a scientist who would be making a presentation there, Dieter Egli, of the New York Stem Cell Foundation, and asked him what he thought the stakes were. Egli is Swiss, and he speaks an accented English both blunt and elegant. He said his goal was to use cells to cure disease, because we are made of cells: tiny, complex, independent ecosystems. Eggs are single cells — the largest ones in the human body — and to him, replacing their mitochondria, then putting them back into women safely and successfully, is like beginner cell therapy. Rather than open the door to eugenics, we might in fact be opening the door to curing degenerative diseases like diabetes, Alzheimer’s and Parkinson’s, even the atrophy of aging. The coming century in medicine will be the century of the cell.