How do you make a baby from three parents? Like this.

The U.K. just became the first country to legalize three-parent babies.  That’s right, three-parent babies. Made when a man and two women who really like each other—wait, what?

The House of Commons approved the law earlier this month and tonight the House of Lords voted similarly (280 to 48), allowing fertility clinics to apply for a special license to use a particular technique. But how are these triple-‘rent babies made?

It’s all about the mitochondria. These are the powerhouses that reside in every cell (except for red blood cells) and convert food into energy. Mitochondria have a tiny amount of their own DNA, separate from the DNA in a cell’s nucleus and only passed down from the mother.

Sometimes there are mutations in this mitochondrial DNA. Mutations don’t affect personality or appearance, but they can lead to diseases for which symptoms range from strokes to dementia to multiple sclerosis-type conditions.  Between 1,000 and 4,000 children are born with a mitochondrial disease every year in the U.S. (Because of misdiagnosis, the number is hard to nail down).

So the three-parent IVF process allows for a woman to donate healthy mitochondria to a woman whose mitochondria have genetic defects. Here are the two basic methods:

The first consists of simply removing the nucleus from a donor’s healthy egg cell and replacing it with the healthy nucleus from a mother’s egg cell.

The other technique is a bit more complicated: Both the mother’s egg and the donor’s egg are fertilized in vitro. The nucleus from the donor’s fertilized egg is removed and then the parents’ nucleus (from the mother’s egg) replaces it.

The upsides are tremendous: The process will enable parents to spare their children a life of disease and early death. The downsides, according to those who opposed the U.K. law, are the potential for “designer babies.” Humans shouldn’t play God, they argue.

But clearly Parliament believes the prevention of disease for millions of people is worth the faint presence of a third party’s DNA in a single person.