Even the hottest stud can only sire so many offspring. To speed up livestock breeding, biologists have created pigs, goats and mice whose testicles are effectively clones of those of a different male, meaning these animals father that male’s offspring.

“It will allow more farmers to get hold of animals with the characteristics they want,” says Bruce Whitelaw at the Roslin Institute in the UK. “Many people did not think this would work.”

The first step in the process is to create host animals …