Fertility
Mules and hinnies have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile.
There are no recorded cases of fertile mule stallions. A few female mules have produced offspring when mated with a purebred horse or donkey. Herodotus gives an account of such an event as an ill omen of Xerxes' conquest of Greece in 480 BC: "There happened also a portent of another kind while he was still at Sardis,—a mule brought forth young and gave birth to a mule" (Herodotus The Histories 7:57).
Since 1527 there have been more than 60 documented cases of foals born to female mules around the world. There are reports that a mule in China produced a foal in 1984. In Morocco, in early 2002, a mare mule produced a rare foal. In 2007 a mule named Kate gave birth to a mule son in Colorado. Blood and hair samples were tested verifying that the mother was a mule and the colt was indeed her offspring.
An 1939 article in the Journal of Heredity describes two offspring of a fertile mare mule named "Old Bec", which was owned at the time by the A&M College of Texas (now Texas A&M University) in the late 1920s. One of the foals was a female, sired by a jack. Unlike its mother, it was sterile. The other, sired by a five-gaited saddlebred stallion, exhibited no characteristics of any donkey. That horse, a stallion, was bred to several mares, which gave birth to live foals that showed no characteristics of the donkey.
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