A Slippery Slope to Human Germline Modification

The UK government’s recent move towards human trials of mitochondrial-replacement techniques has prompted intense interest among scientists and bioethicists, while the media continue to frame mitochondrial replacement as a matter of ‘three-parent babies’. The description is accurate — it would involve a woman affected by mitochondrial disease, whose egg provides a nucleus, a second woman to provide a ‘healthy’ egg and a man to provide sperm — but this simple framing overshadows profound social and ethical concerns.

Mitochondrial-replacement procedures would constitute germline modification. Were the United Kingdom to grant a regulatory go-ahead, it would unilaterally cross a legal and ethical line on this issue that has been observed by the entire international community. This consensus holds that genetic-engineering tools may be applied, with appropriate care and safeguards, to treat an individual’s medical condition, but should not be used to modify gametes or early embryos and so manipulate the characteristics of future children.

Supporters argue that these concerns do not apply to modifications of mitochondrial DNA, which they characterize as an insignificant part of the human genome that does not affect...