THE CONVENTIONAL way of making babies is no secret: a father produces a sperm which, when it comes into contact with a mother’s egg, gives rise to an embryo containing genetic material from both parents. The process is popular but not universally accessible; same-sex couples cannot produce biological children together. Scientists have therefore long sought more ambitious recipes, especially ones that remove the need for either parent. They have enjoyed some success: mice with two mothers and no fathers were reported in 2004. But creating animals with two fathers and no mother has proved much harder.