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In its common modern meaning, a mule is the offspring of a male donkey and a female horse. The reverse, the offspring of a male horse and a female donkey, is called a hinny. The term "mule" (Latin mulus) was formerly applied to the offspring of any two creatures of different species – in modern usage, a "hybrid".
The mule, easier to breed and usually larger in size than a hinny, has monopolized the attention of breeders. The chromosome match-up more often occurs when the jack (male donkey) is the sire and the mare (female horse) is the dam. Sometimes people let a stallion (male horse) run with a jenny (female donkey) for as long as six years before getting her pregnant. Mules and hinnies are almost always sterile (see fertile mules below for rare cases) (see External links). The sterility is attributed to the different number of chromosomes the two species have: donkeys have 62 chromosomes, whereas horses have 64. Their offspring thus have 63 chromosomes which cannot evenly divide.
A female mule, called a "molly", that has estrus cycles and can carry a fetus, can occasionally occur naturally as well as through embryo transfer. The difficulty is in getting the molly pregnant in the first place.
In its short thick head, long ears, thin limbs, small narrow hooves, short mane, absence of chestnuts (horny growths) inside the hocks, and tail hairless at the root, the mule looks like a donkey. In height and body, shape of neck and croup, uniformity of coat, and teeth, it appears equine. It does not sound exactly like a donkey or a horse. Instead, a mule makes a sound that is similar to a donkey's but also has the whinnying characteristics of horse. Sometimes, mules whimper. The coat of mules comes in the same variety as that of horses. However, mules are often Bay or Sorrel due to the type of Jack(Sire) used. Common colors are Sorrel, Bay, Black, and Grey. Less common are White, Roans (both blue and red), Palomino, Dun, and Buckskin. Least common are Paint mules or Tobianos.
The mule possesses the sobriety, patience, endurance and sure-footedness of the ass, and the vigour, strength and courage of the horse. Operators of working animals generally find mules preferable to horses: mules show less impatience under the pressure of heavy weights, whereas their skin, harder and less sensitive than that of horses, renders them more capable of resisting sun and rain. Their hooves are harder than horses, and they show a natural resistance to disease and insects. Many North American farmers with clay soil found mules superior as plow animals, especially in the U.S. state of Missouri, hence the expression "stubborn as a Missouri mule".
Mules are generally less tolerant towards dogs than horses, and are capable of defending their rider against, and even killing, a mountain lion. They are also capable of striking out with any of their hooves in any direction.
 Fertile mules
Several female mules have produced offspring when mated with a purebred horse or ass. Since 1527 there have been more than 60 documented cases of foals born to female mules around the world. Mules and hinnies have 63 chromosomes that are a mixture of one from each parent. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos.
- Cornevin and Lesbre stated that in 1873 an Arab mule in Africa was bred to a stallion and produced female offspring. The parents and the offspring were sent to the Jardin d'Acclimatation in Paris. The mule produced a second female offspring sired by the same stallion and then two male offspring, one sired by an ass and the other by a stallion. The female progeny were fertile, but their offspring were feeble and died at birth. Cossar Ewart recorded an Indian case in which a female mule gave birth to a male colt. The best documented fertile mule mare was "Krause" who produced two male offspring when bred back to her own sire.
- In most fertile mule mares, the mare passes on a complete set of her maternal genes (i.e. from her horse/pony mother) to the foal; a female mule bred to a horse will therefore produce a 100% horse foal. In the 1920s, "Old Beck" (Texas A&M) produced a mule daughter called "Kit". When Old Beck was bred to a horse stallion she produced a horse son (he sired horse foals). When bred to a donkey, she produced mule offspring. Likewise, a mare mule in Brazil has produced two 100% horse sons sired by a horse stallion.
- In Morocco, a mare mule produced a male foal that was 75% donkey and 25% horse i.e. she passed on a mixture of genes instead of passing on her maternal chromosomes in the expected way. There are no recorded cases of fertile mule stallions. There is an unverified case of a mare mule that produced a mule daughter. The daughter was also fertile and produced a horse-like foal with some mule traits; this was dubbed a "hule". There are no reports as to whether the mule was fertile.
- A comparable case is that of a fertile hinny (ass mother, horse sire – the reverse of a mule) in China. Her offspring, "Dragon Foal", was sired by a donkey. Scientists expected a donkey foal if the mother had passed on her maternal chromosomes in the same way as a mule. However, Dragon Foal resembles a strange donkey with mule-like features. Her chromosomes and DNA tests confirm that she is a previously undocumented combination.
 The Modern Mule
After World War II, mules fell on hard times. Mules for farming and transportation of agricultural products gave way to modern tractors and trucks and the number of mules in the Unites States fell to historic lows. A dedicated number of mule breeders, however, continued the tradition as hobby and continued breeding the great lines of mammoth jacks started in the United States by George Washington with the gift from the King of Spain of two Catalan Jacks. These hobby breeders began to utilize better mares for mule production until today's modern saddle mule emerged. Exhibition shows where mules pulled heavy loads have now been joined with mules competing in Western and English Pleasure riding, as well as dressage and hunter jumper competition. There is now even a cable channel show produced by Meredith Hodges of the Lucky Three Ranch dedicated to the training of donkeys and mules. Mules, once snubbed at traditional horse shows have, through the efforts of riders like Meredith Hodges, been accepted for competition at the most exclusive horse shows in the world in all disciplines.
 The Mule Clone
In 2003, researchers at University of Idaho and Utah State University finally found a way to get mules to reproduce - by cloning the first mule as part of Project Idaho.
The research team includes Gordon Woods, UI professor of animal and veterinary science, Kenneth L. White, Utah State University professor of animal science, and Dirk Vanderwall, UI assistant professor of animal and veterinary science.
The baby mule, Idaho Gem, was born May 4. It is the first clone of a hybrid animal. A mule results from a cross between a female horse, a mare, and a male donkey, a jack. As hybrids, mules are sterile, except in extremely rare cases.
Veterinary examinations of the foal and its surrogate mother showed them to be in good health soon after birth.
The foal’s DNA comes from a fetal cell culture first established in 1998 at the University of Idaho.
 Mules of a Different Color
Mules today come in all shapes sizes and colors, from minis under 50 pounds to maxis over 2000 pounds, and in every color combination imaginable. Mules from appaloosa mares produce wildly colored mules, much like their appaloosa horse relatives, but with even wilder skewed colors. The appaloosa color is produced by a complex of genes known as the Leopard Complex (Lp). Mares homozygous for the Lp gene bred to any color donkey will produce an appaloosa colored mule. A few examples of mules born of appaloosa and paint mares are shown here.
Freckles' (left) spectacular color is the result of the full expression of the Lp gene, and his color pattern is called "Leopard". His Leopard pattern is unusual because he is tri-colored, black, brown and white. His color is also impacted by the roaning gene which can be seen at his neck and on his sides as "speckled" coloration. The roaning areas will continue to lighten over time and he may actually become white in the roaning areas. Freckles appaloosa color pattern comes entirely from his mother, who had to have been homozygous for the Leopard gene, expressed as LpLp. We suspect, noting the white socks he sports on his legs, that his mother also carried the Splash gene, or that his donkey sire was spotted or carried the Splash gene.
This baby (shown right) was born in 2006 to a registered paint mare. Her paint coloration, seen only in the broken pattern on her stockings, results from the expression of the "Splash" gene, which is active in many horses but comes to full expression in the Paint breed of horse. Her Paint dam can be seen standing behind her in this photo. She shares the same sire as the three full-sibling mules shown below.
-The mules shown below are full siblings, having the same sire and dam.-
This series of photographs illustrates the variety of colors that can come from the same genetics.
(Photo left) Note Honey's full hip blanket and the expression of the roaning gene on her neck. Also note that there are no white stockings on Honey. White stockings are not a part of the Lp gene complex, but are instead from the expression of the paint splash gene.
Note Honey's brother Showoff (shown in the photo to the right) looks like a cross between a paint and an appaloosa, but he is from the same appaloosa mare. It is unclear as to whether Showoff's white legs are a product of the Lp gene expression turning off the brown color and leaving the resulting white or whether the sire or dam had unexpressed splash gene which came forward.
Honey Too looks just like Honey, and is also roaning. Note the similarity in pattern, but difference in base color. Honey Too's base color is bay. The dam of all three of these appaloosa mules is shown in the background. Note that she has no spotting at all, and her color pattern is known as few spot leopard.
 See also
 External links
- British Mule Society
- A case of a mule giving birth
- LongearsMall.com – Online Donkey & Mule Community and Resource Directory
- Supply Mules in Kashmir
- John Henry - One of the world's nicest mules
- Project Idaho