Starting next year, children in the U.K. may be born with the DNA of three different people, after a Parliamentary vote earlier today approved a controversial fertility procedure.

The lower chamber of British Parliament, the House of Commons, voted 382-128 today to pass a bill authorizing an in-vitro fertilization technique that would combine two parents’ genetic material with that of a third female donor. The upper chamber, the House of Lords, is expected to take up the issue next month; if they also vote in favor, three-person IVF could become legal by October, making the U.K. the first country in the world to permit it.

The procedure would allow women who carry the genes for mitochondrial disease—a collection of inherited and incurable conditions—to have their own biological children without passing down the risk. Mitochondria, often described as the “power center” of the cell, are the organelles that provide it with energy. They also have 37 genes within them—and when those genes have certain mutations, they can trigger a host of debilitating and sometimes fatal problems, including muscular dystrophy, heart and liver issues, seizures, and diabetes. Worldwide, an estimated one out of every 6,500 children is thought to have some form of mitochondrial disease, which can only be passed down from mother to child (sperm doesn’t play a role).

The procedure, pioneered by a team of researchers at the U.K.’s Newcastle University, can be done a couple different ways. In the first, maternal spindle transfer, the nucleus is taken from a mother’s egg and inserted into the donor egg, which has been cleared of everything but its mitochondria. The resulting egg—which contains nuclear DNA from one woman, mitochondrial DNA from another—is then fertilized with the father’s sperm. In the second method, called pro-nuclear transfer, both the mother’s egg and the donor’s egg are fertilized; the mother’s nuclear DNA is then taken out of her egg and inserted into the donor’s, which has had its own nucleus removed.

Last week, a group of 40 researchers, bioethicists, and government advisors from around the world penned an open letter to Parliament in support of the procedure, calling it “an international demonstration of how good regulation helps medical science to advance in step with wider society.”

And on Sunday, an international collection of advocacy groups—including the U.S.-based United Mitochondrial Disease Foundation—co-authored a similar letter to British lawmakers, describing mitochondrial disease as “unimaginably cruel.” Three-person IVF, they said, “offers families the first glimmer of hope that they might be able to have a baby that will live without pain and suffering.”

On the other side of the issue are those who view three-person IVF as the first step in a slippery slope of genetic engineering. As Olga Khazan pointed out in July,

Many find the mitochondrial procedure morally questionable because of how close it seems to playing God, or Nature, or Whoever you think is in charge of making kids. Penetrating the inside of a cell and tampering with its contents is, at best, controversial, and at worst, "walking in Hitler’s footsteps," as one angry letter to the FDA put it. Some worry it's in the same sci-fi realm as "designer babies."

“There is a very clear boundary that babies cannot be genetically altered, and that once you’ve decided that you can, even for a small number of genes, you have done something very profound and then it's merely a matter of degree as to what you do next,” conservative lawmaker Jacob Rees-Mogg, who opposed the bill, told the BBC after today’s vote.

But scientists argue that the “designer baby” worries are unfounded, as mitochondrial DNA can’t really design much of anything. “It is not part of what makes us genetically who we are,” reproductive ethicist Gillian Lockwood told the BBC. “It doesn’t affect height, eye color, intelligence, musicality.”

Lockwood added that the term “three-parent baby” is a misnomer; around one-tenth of one percent of a child’s DNA would come from the donor, with the mother and father supplying the rest. "The biggest problem is that this has been described as three-parent IVF. In fact, it is 2.001-parent IVF,” she said.

Others have pointed out that three-person IVF is not the only medical procedure to involve the genetic material of another person. “Let’s say unfortunately you have leukemia and you have your bone marrow radiated for the cancer to be killed, and then it is replaced by bone marrow from someone else—say, me,” Peter Braude, an emeritus professor of gynecology at Kings College London, told the BBC last year. “You won’t be related to me … but you will have DNA from a third person circulating in your body.”

In the U.S., where around 1,000-4,000 children are born with mitochondrial disease each year, a similar debate over three-person IVF is beginning to take shape. The Food and Drug Administration convened a committee last February to discuss whether or not to permit clinical trials for three-person IVF, though no decision was reached. At the FDA’s request, the Institute of Medicine has begun work on a “consensus report regarding the ethical and social-policy issues” of the procedure, with the first of five meetings taking place last month.

Even if the U.K. law takes effect, the babies it helps bring into being won’t be the first generation to carry the genes of three separate people. In the 1990s, a team of researchers at the St. Barnabus Institute in New Jersey developed an infertility treatment called cytoplasmic transfer, which involved inserting donor cytoplasm, the substance within a cell that contains the mitochondria and other organelles, into a woman’s egg. The FDA banned the procedure in 2002—but not before several other clinics had mimicked the New Jersey team’s methods, helping to produce an estimated 30-50 children whose genetic material includes traces of mitochondrial donor DNA.

The St. Barnabus Institute began a follow-up study last year to investigate the long-term effects of cytoplasmic transfer, according to the BBC.

But leading up to today’s debate, Lisa Jardine, the former chair of the U.K.’s Human Fertilisation and Embryology Authority, called safety concerns about three-person IVF “a red herring.”

"All of those issues have been investigated," she said. "The scientific committees have said there is no evidence this procedure is unsafe, but like all good scientists, they say it will require careful progress."