It has been a long-standing rule in fertility treatment that no
scientist should attempt to modify the genes of a human embryo if that
modification can be passed on to subsequent generations through the
“germline” – the eggs and sperm of the future person. Now Parliament is
about to consider another form of germline modification, so-called
“three-parent embryos”, this time involving the genes of the
mitochondria, described as the tiny “power packs” of the cell, which
exist outside the nucleus.