A boy is the world’s first baby that has the DNA of three people. The “three parent” technique allows parents with rare genetic mutations to have healthy babies. In this case, the mother carries genes for Leigh syndrome in her mitochondrial genome. She is healthy but had lost her first two children to the disease which affects the brain, muscles and nerves of developing infants.
There are two approaches to overcome this. One involves fertilising both the mother’s egg and a donor egg with the father’s sperm. Before the fertilised eggs start dividing into early-stage embryos, each nucleus is removed. The nucleus from the donor’s fertilised egg is discarded and replaced by that from the mother’s fertilised egg. But, for religious reasons, the parents were opposed to the destruction of embryos. So, another technique was used: spindle transfer. The nucleus from one of the mother’s eggs was removed and inserted into a donor egg that had had its own nucleus removed. The resulting egg – with nuclear DNA from the mother and mitochondrial DNA from a donor – was then fertilised with the father’s sperm. Five embryos were obtained this way, only one of which developed normally. This embryo was implanted in the mother. It was a male embryo, so that the resulting child wouldn’t pass on any inherited mitochondrial DNA.
Because neither method has been approved in the US, the procedure was performed in Mexico instead. A remaining concern is safety. The last time the creation of an embryo using DNA from three people was attempted was in the 1990s; some of the babies developed genetic disorders, and the technique was banned. The problem may have arisen from the babies having mitochondria from two sources.
The boy’s mitochondria were tested and it turned out that less than 1% carry the mutation. Hopefully, this is too low to cause any problems; generally it is thought to take around 18% of mitochondria to be affected before problems start.