Mules are the offspring of a male donkey and a female horse. The reason why mules are sterile is because they have a genetic phenomenon called "hybrid sterility."
"Hybrid sterility" means that in some species, hybrids between different species (that is, the offspring produced by mating between two different species) are usually unable to reproduce. This is a reproductive disorder due to genomic differences between species. Although donkeys and horses can form hybrids to some extent, mules have difficulty reproducing because their paternal line is donkey and their dam is horse.
Specifically, mules' infertility mainly results from a mismatch in the number of chromosomes in their germ cells. Donkeys have 62 chromosomes, while horses have 64 chromosomes. When male donkeys and female horses mate, their offspring, mules, inherit 63 chromosomes. This mismatch in chromosome numbers leads to reproductive infertility.
This hybrid sterility phenomenon not only occurs in mules, but also commonly occurs in the hybrid offspring of other species, such as wolf-dog hybrids, tiger-lion hybrid ligers, etc. This is a mechanism in nature that protects purity between species and avoids excessive gene flow and mixing between different species.
