Britain moved a step closer Tuesday to becoming the first country to legalize so-called three-parent babies, as lawmakers moved closer to allowing fertility labs to use genetic material from a mother, father and a female donor.

The debate in the House of Commons reflected the deep passions on both sides of the issue of allowing in vitro procedures that could prevent the passing on of inherited and incurable diseases through mitochondrial DNA, which is carried from mother to child.

Supporters say changing the law would offer hope to couples who otherwise would risk transferring diseases such as muscular dystrophy to their offspring. But a wide range of critics question the ethics of the proposal, saying it would be another step toward creating “designer babies.”

The proposal passed, 382 to 128, and moved to Parliament’s upper chamber, the House of Lords, where a vote is expected soon, although no date has been set. Commentators say the first three-parent baby could be born next year if the legislative effort succeeds.

“This is a bold step for Parliament to take, but it is a considered and informed step,” the parliamentary undersecretary for public health, Jane Ellison, said before the vote. “For many families affected, this is, indeed, that light at the end of the dark tunnel,” she added

How one baby can have three people’s DNA

The technique, called mitochondrial donation, involves replacing a woman’s faulty mitochondrial DNA with healthy DNA from a female donor. Even if the bill passes, couples seeking treatment would need the permission of the Human Fertilization and Embryology Authority, Britain’s fertility regulator.

Religious leaders have voiced concerns, warning the government against rushing into a decision and arguing that more research is needed.

Fears over genetic manipulation are not unique to Britain. In the 1990s, the United States pioneered a technique called cytoplasmic transfer, aimed at helping infertile women to have their own babies. As part of the procedure, donor material containing mitochondrial DNA was added to the mother’s egg. Fewer than 100 babies were born before the Food and Drug Administration effectively banned the practice over medical and ethical concerns.

If the British procedure gets the green light, a baby conceived through the technique would receive its key genetic material from its mother and father, and just a small amount of DNA from a donor female, who would remain anonymous.

The aim is to help the estimated 2,473 women in Britain who are at risk of passing on possibly disease-triggering mitochondrial DNA to their children.

Mitochondria are tiny structures inside cells that act like batteries, providing energy to the cells. They also have their own DNA. When they do not function properly, the results can be devastating. Faulty mitochondria have been linked to blindness, deafness, dementia and muscular dystrophy.

A mitochondrion’s 37 genes are a small fraction of those in the human genome, and unlike nuclear DNA, mitochondrial DNA does not affect appearance characteristics such as hair color. The ban on tinkering with nuclear DNA would remain in place.

A scientist works during an in-vitro fertilization process. (Ben Birchall/AP)

Still, should the law change, babies could be born with a very small fraction of their DNA, about 0.2 percent, from a female donor. Some opponents of the proposed legislative change speak of Britain’s stepping on to a slippery slope.

Fiona Bruce, a member of Parliament from the Conservative Party, said the move would be tantamount to letting “the genie out of the bottle.”

The bulk of the British scientific community, however, appeared to welcome the result of the vote and widely interpreted it as providing clear guidelines on what is acceptable.

“I don’t agree with the slippery slope argument at all,” said professor Alison Murdoch of Newcastle University, which has been at the forefront of mitochondrial research in Britain. “By having a legal framework that says you can do this but you can’t do the other without criminal sanctions, there is an absolute barrier.”