The sterility of a mule is accredited to the difference in the number of chromosomes that a donkey and a horse possess. That is, while horses have 64 chromosomes, donkey's posses just 62. The mule, their offspring as a result have 63 chromosomes and these cannot evenly divide. However it is important to note that there are exceptions to the rule and there have been documented cases where female mules have succeeded in producing offspring after they were mated to either a purebred horse or ass.
The collective noun for mules is a barren of mules. They are also often called a rake of mules.
It is interesting to note that, in general, mules are thought to be more intelligent than horses and donkeys.
A horse and a donkey are from the same genus, but not the same species.
There are many wild horses and burros in the western U.S., and some people wonder why there are no mules. This is because they are entirely the product of artificial insemination, between a stallion and a jenny (female donkey). Mules may be sterile, but they have many advantages over horses or donkeys. They are vastly more intelligent than either species. They are also much more sure of foot, hardy, and can carry stronger loads. As for their alleged oneriness, that is an indication of their intelligence, according to William Faulkner: "A mule will work patiently for you for 10 years for the opportunity to kick you just once." In hardrock mines in the western U.S., mules spent their entire lives underground (thus becoming blind) hauling ore carts. They memorized the bell system indicating the position of the ore lifts, and knew exactly where to position the carts. Miners rewarded them with the animals' favourite treat: A chew of tobacco. But if a mule took a dislike to a miner, it would pin him against the tunnel wall. Many outfitters in the southern Sierra Nevada range of California offer pack trips with mule trains. You can also ride a mule down from the South Rim of the Grand Canyon to Phantom Ranch on the Colorado River.