In August 1996, at St. Barnabas Medical Center in Livingston, N.J., a 39-year-old mechanical engineer from Pittsburgh named Maureen Ott became pregnant. Ott had been trying for almost seven years to conceive a child through in vitro fertilization. Unwilling to give up, she submitted to an experimental procedure in which doctors extracted her eggs, slid a needle through their shiny coat and injected not only her husband’s sperm but also a small amount of cytoplasm from another woman’s egg. When the embryo was implanted in Ott’s womb, she became the first woman on record to be successfully impregnated using this procedure, which some say is the root of an exciting medical advance and others say is the beginning of the end of the human species. The fresh cytoplasm that entered Ott’s eggs, contained mitochondria: bean-shaped organelles that power our cells like batteries. But mitochondria also contain their own DNA, which meant that her child could possess the genetic material of three people. In fact, the 37 genes in mitochondrial DNA pass directly from a woman’s egg into every cell of her offspring, including his or her germ cells, the sperm or eggs that eventually produce the next generation, so if Ott had a girl and the donor mitochondria injected into Ott’s egg made it into the eggs of her daughter, they could be passed along to her children.

Mitochondrial DNA wasn’t discovered until the 1960s, and it wasn’t until 1988 that two high-profile papers, published by groups at the Institute of Neurology in London and Emory University School of Medicine, revealed that mutations in mtDNA can cause disease. Subsequent research has identified hundreds of mitochondrial diseases, largely related to impaired energy production in cells, that are incurable and can affect any system in the body, resulting in deafness, blindness, muscle weakness, cognitive impairments, heart, lung and kidney failure, diabetes and death. About 1 in 4,000 children and adults is diagnosed with mitochondrial disease, but because symptoms are so varied, doctors think many more cases are misdiagnosed. 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.