MRT swaps out defective mitochondria passed down from mother to child with healthy versions from another female donor. Mitochondria provide energy to all living cells, and faulty ones can cause babies to develop degenerative diseases. The "three-parent" baby thus inherits most of its DNA and traits from the mother and father, with the third person supplying only .2 percent of the kid's genes, none of which control characteristics like hair or eye color.
The Newcastle team showed that MRT worked in 2010 and have been refining their method since. In early experiments, faulty mitochondria still ended up transferring. But of the 200 successful MRT procedures, 79 percent of the test embryos have had less than 2 percent faulty mitochondria (children with less than 30 percent typically don't develop degenerative diseases). The method isn't perfect: compared to normal IVF embryos, some cells extracted from embryos with replaced mitochondria showed increased levels of defective DNA as the cells multiplied. And since we won't know how MRT will affect children over time, the Newcastle scientists will likely only test the method on male babies for a full generation (only women can pass on defective mitochondria).
The UK Parliament permitted tests on human donor eggs in February 2015 amid protest by the Church of England and others. A year later on this side of the pond, a committee of bioethicists convened by the US Food and Drug Administration cautiously approved of MRT, but a congressional ban remains: Lawmakers snuck a provision in the 2016 Congressional budget preventing federal funding of experiments that alter human embryos. Until a new budget bill passes, it falls to countries without such restrictions to develop treatments that could help the roughly 1 in 4000 babies born with defective mitochondria.