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- The word hinny is also a term of endearment used in North East England, equivalent to honey; see Geordie and Mackem.
A hinny is a domestic equine hybrid that is the offspring of a male horse and a female donkey. It is similar to the more common mule, which is the product of a female horse and a male donkey.
Physical description 
Hinnies are on average slightly smaller than mules. There is much speculation about the size variances between the two types of hybrids. Some fanciers feel this size difference is merely physiological, due to the smaller size of the donkey dam, as compared with a much larger mare. Others claim it is genetic, but the view of the American Donkey and Mule Society is, "The genetic inheritance of the hinny is exactly the same as the mule."
Thus, hinnies are smaller because donkeys are, for the most part, smaller than horses, and growth potential of equine offspring is influenced by the size of the dam's womb. However, like mules, hinnies do come in many sizes. This is because donkeys come in many sizes, from miniatures, as small as 24 inches (610 mm) at the withers, to Mammoth jacks and jennies that may be over 15 hands (60 inches, 152 cm) 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 tallest breed of horse. There are some very large mules, mostly from work horse breeds such as the Belgian.
Other than size, there are some minor differences that occur frequently between mules and hinnies. The head of a hinny resembles that of a horse, more so than mule heads. Hinnies often have shorter ears, although they are still longer than those of horses, and more horse-like manes and tails than mules. Certain traits, such as the popular gait that some horses and donkeys possess, seem to pass more readily through the male parent. Therefore, many people have tried to produce gaited hybrids by using gaited male horses with female donkeys, in hopes of creating gaited hinnies.
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 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."
Male hinnies and mules are usually castrated to help control their behavior by eliminating their interest in females. The male hinny or mule can and will mate, but the emission is not fertile.
Female hinnies and mules are not customarily spayed, and may or may not go through estrus. Female mules have been known to produce offspring when mated to a 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.
In China in 1981, a hinny mare proved fertile with a donkey stallion. When the Chinese hinny was bred to a donkey jack, she produced "Dragon Foal," who resembled a donkey with mule-like features.
In Morocco in 2003, a mule mare bred to a donkey stallion produced a male foal; DNA testing revealed the foal has a mixed karyotype hybrid like the Chinese hinny offspring, Dragon Foal.
There are other reasons for the rarity of hinnies. Female donkeys, jennies, and male horses, stallions, are choosier about their mates than horse mares and donkey jacks. Thus, the two parties involved may not care to mate. Even if they do cooperate, female donkeys are less likely to conceive when bred to a horse than horse mares are when bred to a donkey. Breeding large hinnies is an even bigger challenge, as it requires stock from a jenny of Mammoth donkey (Baudet de Poitou or American Mammoth Jack. 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 females are in high demand to produce fertile pure-bred Mammoth foals.
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