In its common moern meaning, a mule is the offspring of a male donkey and a female horse, which is classified as a kind of F1 hybrid. 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 infertile offspring of any two creatures of different species. 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 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.