British lawmakers passed a bill on Tuesday allowing scientists to combine the DNA of three people to create a child free of previously unavoidable genetic issues passed down from the mother. The United Kingdom is the first country to allow the procedure that some call “three-parent in vitro fertilization.” The process involves transplanting a healthy nucleus from a mother who suffers from mitochondrial defects into an healthy egg of a surrogate mother that’s had its nucleus removed, and then transferring that healthy fertilized egg back into the “traditional” mother.

Proponents say it’s a medical breakthrough that allows families to end maternally inherited health issues that have existed in their maternal lines forever; critics call it an effort to make “designer babies” free of any health issues.

Parkinson’s disease and mitochondrial dysfunction, which can lead to the accelerated breakdown of heart, brain and muscle tissue, are two conditions that could be removed from the maternal line with the procedure. Mitochondrial diseases currently cannot be cured otherwise.

Mitochondrial diseases affect one in 6,500 children worldwide, according to Reuters.

A child conceived using the procedure would have the DNA of his father, mother and the surrogate mother who “donated” her healthy mitochondria to the zygote, which would be passed on for the rest of the maternal line of that family, because a child’s mitochondrial DNA is almost always exclusively inherited from the mother. For the most part, replacing the mitochondria will have little to no effect on the child’s “traits,” because mitochondria only produce energy for the cells. Still, some people are uneasy about the procedure. This video from the Human Fertilisation & Embryology Authority of the U.K.’s Department of Health explains how it works:

Around 99 percent of the DNA of American teen Alana Saarinen, a “three-parent baby” born in 2000, was found to have been passed down from her parents, while only 1 percent came from her surrogate mother. You can read more about Saarinen and her procedure here, via BBC. The U.S. Food and Drug Administration banned the procedure a year after she was born.

Some supporters of a ban on the procedure argue it is unethical, while other say the science isn’t in on whether it’s ultimately safe. Sheldon Krimsky, a philosopher at Tufts University in Medford, Massachusetts, asked in a 2013 debate what would happen if the genetic modification produced a more serious medical complication, or if the parents didn’t get what they wanted? Could they just “discard the results when they don’t work out?” he asked.