Why might a mule be sterile?
Mules are a cross between horses and donkeys. Horses have 64 chromosomes and donkeys have only 62 chromosomes. The two are similar enough to breed but as a result the foal (being a mule) will only have 62 full chromosomes and one half of a chromosome (offspring gets half of moms and half of dads chromosomes). This effects the genes that control reproduction.