The prospect of a human baby with three biological parents has moved closer after scientists created monkeys using a technique that one day could stop children from inheriting severe genetic diseases.

The birth of four healthy macaque monkeys in the U.S. offers the strongest evidence yet that DNA can be transplanted safely from one egg to another to correct genetic defects that damage health.

The successful experiment in a close human relative suggests that it should be possible within a few years to use the method to help women who carry genetic disorders to avoid passing them to their children.

It should allow scientists to replace faulty “cellular batteries” called mitochondria, which affect about 1 in 6,500 births. While most mitochondria defects have mild effects, some can trigger severe brain, heart, muscle and liver conditions, as well as cancer, diabetes, blindness and deafness.

The technique is controversial, however, because the children it creates would inherit genetic material from three parents. The mother and father would contribute most of their child’s DNA but a small amount would come from a second woman donating healthy mitochondria.

Such children would be the first produced by germline genetic engineering, in which genes introduced by artificial means would be passed to successive generations.

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