Closely related species in the genus Equus can be bred to produce offspring that are viable and robust, but almost always sterile.

A female horse (Equus caballus) bred with a male donkey (Equus asinus) will yield a Mule...

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A male horse (Equus caballus) bred with a female donkey (Equus asinus) will yield a Hinny...

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A donkey (Equus asinus) bred to a zebra (E. burchelli, E. grevyi, or E. zebra) yields a "Zeedonk"...

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Each species has a distinct chromosome number:

The hybrids are viable because their genes--housed on chromosomes that appear to have undergone major physical rearrangement (evident in the synteny of their chromosomes) during the adaptive radiation of Equus species--are largely homologous. They have all the necessary genetic information encoding normal devlopment and body function. This can be shown via chromosomal hybridization in which chromosomes from different species are allowed to pair as if during metaphase.

However, because the chromosomes have changed so much during Equus evolution, the chromosomes cannot pair properly during meiosis to allow crossing over and successful segretation of homologs into new daughter cells. Hence, the hybrids are almost always sterile, as they cannot produce viable gametes.