In early 2018, Dr. He Jiankui, a (then) 34-year-old Chinese biophysicist, traveled to an Arizona science conference to meet with one of his heroes, the Nobel Prize-winning geneticist James Watson, a man who first proposed the double helix structure of DNA molecules back in 1953. Dr. He was in the midst of a big experiment of his own, genetically engineering embryos to eliminate their risk from disease. It was something that hadn’t been attempted before (or since), and He was having reservations. “Do you think that that’s a good thing to do?” he asked Watson, writing down the question because...