Hinnies are the reciprocal cross to the more common mule. Comparatively, the average hinny has a smaller stature, shorter ears, stronger legs, and a thicker mane than the average mule. The distinct phenotypes of the hinny and the mule are partly attributable to genomic imprinting—an element ofepigenetic inheritance. Physiological arguments for the differing stature of the hinny and the mule cite the smaller womb of the female donkey versus the larger womb of the female horse. Growth potential of equine offspring may be influenced by the size of the dam's womb. The American Donkey and Mule Society appears to interpret these differences as wholly physiological, stating: "The genetic inheritance of the hinny is exactly the same as the mule." Be that as it may, the epigenetic inheritance of the hinny is not the same as the mule, as "the differences between the mule and the hinny are now known to be caused by genomic imprinting, whereby the expression of a gene is determined by its origin rather than its DNA sequence". Like mules, hinnies express broad variation in stature. This is because donkeys come in many sizes, from miniatures, as small as at the withers, to American mammoth donkeys that may be over at the withers. Thus, a hinny is restricted to being about the size of the largest breed of donkey. Mules, however, have a female horse as a parent, so they can be as large as the size of the largest breed of horse, such as those foaled from work horse mares such as the Belgian. Physical differences between hinnies and mules are not restricted to stature. The head of a hinny is said to resemble that of a horse more than it does a mule's, with shorter ears, and more horse-like manes and tails than mules. Beyond the physiological, hinnies and mules differ in temperament despite sharing nuclear genomes. This, too, is believed to be attributable to the action of imprinted genes. A male hinny is properly called a horse hinny, and a female hinny is properly called a mare hinny, though in British English both female hinnies and female donkeys are sometimes called jennets.
Fertility, sterility and rarity
Hinnies are difficult to obtain because of the differences in the number of chromosomes of the horse and the donkey. A donkey has 62 chromosomes, whereas a horse has 64. Hinnies, being hybrids of those two species, have 63 chromosomes and are in the majority of cases sterile. The uneven number of chromosomes results in an incomplete reproductive system. According to the ADMS: "The equine hybrid is easier to obtain when the lower chromosome count, the donkey, is in the male. Therefore breeding for hinnies is more hit-and-miss than breeding for mules." The male hinny or mule can and will mate, but the emission is not fertile. Male hinnies and mules are usually castrated to help control their behavior by eliminating their interest in females. Female hinnies and mules are not customarily spayed, and may or may not go through estrus. Female mules have been known, on rare occasions, to produce offspring when mated to a horse or donkey, although this is extremely uncommon. Since 1527, sixty cases of foals born to female mules around the world have been documented. In contrast, according to the ADMS, there is only one known case of a female hinny doing so. Namely, in China, in 1981, a hinny mare proved fertile with a donkey sire. When the Chinese hinny was bred to a jack, she produced the so-called "Dragon Foal", which resembled a donkey with mule-like features. In Morocco, in 2002, a mule mare bred to a donkey sire produced a male foal. DNA testingrevealed the foal has a mixed karyotype hybrid like the Chinese hinny offspring "Dragon Foal". Hinnies are rare for many other reasons. Donkey jennies and horse stallions can be choosier about their mates than horse mares and donkey jacks. Thus, the two parties involved may not even care to mate. Even if they do cooperate, donkey jennies are less likely to conceive when bred to a horse stallion than horse mares are when bred to a donkey jack. Breeding large hinnies is an even bigger challenge, as it requires stock from a jenny of large size, such as the Baudet de Poitou or American Mammoth Donkey. Mammoth donkey stock is becoming increasingly rare and has been declared an endangered domestic breed. Fanciers are unlikely to devote a Mammoth jenny's valuable breeding time to producing sterile hinny hybrids, when Mammoth jennies are in high demand to produce fertile purebred Mammoth foals.
