Why a Mule Usually Cannot Have Offspring
A horse has 64 chromosomes; a donkey has 62. A mule inherits one set from each parent, giving it 63—an odd number that makes reproduction difficult. The problem appears when the body tries to make eggs or sperm: chromosomes normally pair up and separate into balanced sets, but a mule’s horse and donkey chromosomes often cannot find suitable partners.
Without balanced reproductive cells, fertilization usually cannot produce a viable embryo. That is why mules are almost always sterile, even though they can grow up healthy and work normally.
The Rare Exception
“Sterile” is not quite absolute: rare female mules have produced offspring, while male mules are generally sterile. These unusual cases do not change the usual outcome. The mismatch in chromosome pairing makes successful reproduction an exception, not the rule—and helps explain why mules remain hybrids rather than forming a reliably self-reproducing population.
