Nature Finds a Way

Eye on DNA reports on a mule who gave birth to a...mulette?

If a horse breeds with a donkey, they end up with sterile offspring that have 63 chrosomes - 32 chromsomes from the horse parent and 31 from from the donkey parent. Horse-donkey offspring aka mules are sterile because their odd number of chromosomes makes it technically difficult for chromosomes to pair up properly during the process of meiosis (cell divison of sperm and eggs). This should mean that mules cannot reproduce.
A female mule in Colorado has beaten insurmountable odds and given birth to a foal.