Donkey of Asiatic wild ass or onager, "Equus hemionus", and three subspecies of the kiang, "Equus kiang", of the Himalayan upland. A male donkey (jack) can be crossed with a female horse to produce a mule. A male horse can be crossed with a female donkey to produce a hinny. Horse-donkey hybrids are almost always sterile because horses have 64 chromosomes whereas donkeys have 62, producing offspring with 63 chromosomes. Mules are much more common than hinnies. This is believed to be caused by two factors, the first being proven in cat hybrids, that when the chromosome count of the male
Domestication of the horse less than expected based on the overall genetic variation in the remaining genetic material. This indicates that a relatively few stallions were domesticated and that it is unlikely that many male offspring originating from unions between wild stallions and domestic mares were included in early domesticated breeding stock. Genes located in the mitochondrial DNA are passed on along the maternal line from the mother to her offspring. Multiple analyses of the mitochondrial DNA obtained from modern horses as well as from horse bones and teeth from archaeological and palaeological finds consistently shows an increased genetic diversity in the mitochondrial DNA