When animals of two different species mate, their hybrid offspring can be unhealthy or sterile. Sexual differences in fertility follow a pattern known as Haldane’s Rule, which states that hybrids are afflicted more when they inherit two different sex chromosomes. In mammals, males have XY sex chromosomes, so male “ligers” and “tigons” (offspring between tigers and lions) are sterile, while females, which have two X chromosomes, tend to be more fertile. But in butterflies as well as birds, females have ZW sex chromosomes while males have ZZ, so according to Haldane’s Rule, it is females that are sterile. “Tianzhu has shown it is the fraction of the Z chromosome that matters, not whether you’ve got a particular problem on one region of the chromosome,” Mallet said.