A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybirds between these two species, a mule is easier to obtain than a hinny (the offspring of a male horse and a female donkey). All male mules and most female mules are infertile.
The size of a mule and work to which it is put depends largely on the breeding of the mule's dam. Mules can be lightweight, medium weight, or even, when produced from draught horse mares, of moderate heavy weight.
The mule, easier to breed and usually larger in size than a hinny, has monopolized the attention of breeders. Reproductive success occurs more often when a donkey is the sire and the horse is the dam. Sometimes people let a stallion (male horse) run with a jenny (female donkey) for as long as six years before she becomes pregnant. Mules and hinnies are almost always sterile (see fertile mules below for rare cases). The sterility is attributed to the differing number of chromosomes of the two species: donkeys have 62 chromosomes, whereas horses have 64.