Reproductive isolation from a cross between a horse and a donkey or between a mare and a donkey, respectively. These animals are nearly always sterile due to the difference in the number of chromosomes between the two parent species. Both horses and donkeys belong to the genus "Equus", but "Equus caballus" has 64 chromosomes, while "Equus asinus" only has 62. A cross will produce offspring (mule or hinny) with 63 chromosomes, that will not form pairs, which means that they do not divide in a balanced manner during meiosis. In the wild, the horses and donkeys ignore each other and do not
Onager known as the Persian onager. Until this day, the species share the same name, "onager". The onager is a member of the subgenus "Asinus", belonging to the genus "Equus" and is classified under the family Equidae. The species was described and given its binomial name "Equus hemionus" by German zoologist Peter Simon Pallas in 1775. The Asiatic wild ass, among Old World equids, existed for more than 4 million years. The oldest divergence of "Equus" was the onager followed by the zebras and onwards. A new species called the kiang ("E. kiang"), a Tibetan relative, was previously considered to be