Baby embryo is curled into a ball. The picture has been filtered with a red tint.

The terms "genetically modified babies" and "designer babies" are attention-getters. But beyond the catchy sound bites, what do they really mean - and are they something we need to worry about?

Unfortunately, with the technical capacity to engineer inheritable traits growing quickly, and with the United Kingdom possibly on the verge of loosening its law in order to allow a limited form of inheritable genetic (germline) modification, there is ample reason for concern.

The proposed policy change in the UK would permit licensed fertility clinics to use a biologically radical technique referred to by terms including "mitochondrial replacement," "nuclear genome transfer," and "three-person IVF." This procedure would produce modifications in every cell of any resulting children, and in subsequent generations as well. In this article, we will use the term "nuclear genome transfer," as it is the most technically accurate of the various terms.

The technique is proposed as a way for women affected by a particular subset of severe mitochondrial disorders to have children who are not affected and who are mostly genetically related to them. The researchers promoting...