A mule is the offspring of a male donkey (jack) and a female horse (mare). Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny, which is the product of a female donkey (jenny) and a male horse (stallion).The size of a mule and work to which it is put depend largely on the breeding of the mule's dam (female parent).