Donkeys can reproduce with other donkeys or with horses. When donkeys and horses are bred together, they produce mules or hinnies, which are almost always sterile. There have been a few documented cases of mules reproducing, but it's extremely rare.
A mule is the offspring of a male donkey and a female horse, while a hinny is the offspring of a female donkey and a male horse. According to geneticist Monica Rodriguez, mules and hinnies are infertile because donkey and horse chromosomes do not match. Since offspring inherit half their chromosomes from each parent, the dissimilar chromosomes do not allow mules and hinnies to produce the sperm or eggs necessary to reproduce.