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 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.  A
female mule that has estrus cycles and can carry a fetus is
called a "molly" and can occasional