In its common modern meaning, a
mule is the offspring of a male
donkey and a female
horse, which is classified as a kind of
F1 hybrid. The much rarer 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[
citation needed] and usually larger in size than a hinny, has monopolized the attention of breeders[
citation needed]. 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.
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.