http://en.wikipedia.org/wiki/Hinny
Here’s a fascinating article about hinnies, which are essentially mirror image mules. The animal is new to me, and the genetics involved are pretty mind-blowing. (Real-life chimerae?!) Just be warned that extended reading will start to sound like a Newsradio sketch.
* 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. Instead of a simple hybrid, Dragon Foal is a genetic chimera.