http://en.wikipedia.org/wiki/HinnyGot interested in the topic from Keegans "History of Warfare", wherein he discusses the "cavalry revolution", and the differences among various equiids.
"Female hinnies and mules are not customarily neutered, and may or may not go through estrus. Female mules have been known to produce offspring when mated to a purebred horse or donkey, though this is extremely uncommon. Since 1527 there have been more than sixty documented cases of foals born to female mules around the world. In contrast, according to the ADMS, there is only one known case of a female hinny doing so, and she produced a mix which has, thus far, only been documented once among the offspring of female mules:
* Mule mares pass along 100% of their maternal genes to their offspring, rather than a mix. Since a mule's mother is a horse, as a rule mule mares pass genes which are 100% horse to their foals. Thus, a mule mare bred to a horse stallion will produce a foal which is 100% horse, with no donkey genes at all.
* In China in 1981, a hinny mare proved fertile with a donkey stallion. Scientists expected a 100% donkey foal if the female hinny had passed on her maternal chromosomes the same way as female mules do. However, when the Chinese hinny was bred to a donkey jack, she produced "Dragon Foal," who resembles a donkey with mule-like features. Dragon Foal's chromosomes and DNA tests confirmed that she was a previously undocumented combination, not 50% donkey, from her donkey father, and 50% horse, from her hinny mother. Instead of each gene pair being donkey-horse, with one left over, Dragon Foal has a combination of gene pairs that are donkey-horse and donkey-donkey, with one left over. Her hinny mother passed along a combination of horse and donkey genes.
* In Morocco in 2003, a mule mare bred to a donkey stallion produced a male foal that was 75% donkey and 25% horse, rather than the expected 50%/50% if the mule mare had passed on her maternal chromosones, which are 100% horse, in the usual way. DNA testing revealed the Moroccan foal is a mixed karyotype hybrid like the Chinese hinny offspring, Dragon Foal. This means that, unlike regular hinnies, whose 63 chromosomes consist of 31 pairs that are horse-donkey with one left over, the Morocco colt has about 23 pairs of chromosomes that are donkey-donkey, eight pairs that are horse-donkey, and one left over.
* Because of the mix of gene pairs in the Moroccan foal, it's unknown whether Dragon Foal's genetic oddity is due to her mother being a hinny rather than a mule, or if there is some other factor that applies equally to Dragon Foal in China and the 2003 colt in Morocco.
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