Most people use the word "mule" for any donkey-horse hybrid, but there are actually two distinct crosses:
The most frequently asked question regarding "horse mating donkey" is: Why are mules sterile? Horse Mating Donkey
To conceive, parents must produce sperm and egg cells via meiosis—a process where chromosomes pair up perfectly. In a horse (64 chromosomes), the 32 pairs find their match easily. In a donkey (62 chromosomes), the 31 pairs do the same. Most people use the word "mule" for any
However, when a mule (63 chromosomes) tries to reproduce, the cells contain odd pairs. The 31 donkey chromosomes cannot align perfectly with the 32 horse chromosomes. They form "univalents" (chromosomes without a partner). Consequently, the mule’s body cannot produce viable sperm or eggs. The Chromosome Gap: The primary barrier to reproduction
The Exception: While extremely rare (roughly 1 in 10 million), female mules (mollies) have occasionally produced offspring when mated with a purebred stallion or jack. Documented cases exist in China and Morocco, but these are genetic anomalies.
To understand the outcome of a horse mating a donkey, you must first know the parents.
The Chromosome Gap: The primary barrier to reproduction is this difference in chromosome count. Horses have 64, donkeys have 62. When they mate, the offspring receives 32 from the horse and 31 from the donkey, totaling 63 chromosomes. This odd number is the root of why mules are almost always sterile.