The emergence of a powerful gene-editing technology, known as CRISPR–Cas9, has elicited furious debate about whether and how it might be used to modify the genomes of human embryos. The changes to their genomes would almost certainly be passed down to subsequent generations, breaching an ethical line that has typically been considered uncrossable.
But emerging technologies are already testing the margins of what people deem acceptable. Parents today have unprecedented control over what they pass on to their children: they can use prenatal genetic screening to check for conditions such as Down’s syndrome, and choose whether or not to carry a fetus to term. Preimplantation genetic diagnosis allows couples undergoing in vitro fertilization to select embryos that do not have certain disease-causing mutations. Even altering the heritable genome — as might be done if CRISPR were used to edit embryos — is acceptable to some. Mitochondrial replacement therapy, which replaces a very small number of genes that a mother passes on with those from a donor, was approved last year in the United Kingdom for people who are at risk of certain genetic disorders.