The offspring of a horse and a donkey is most commonly known as a mule, a robust and highly valued equine hybrid. Mules are the result of a cross between a male donkey, called a jack, and a female horse, known as a mare. The mule is a hybrid animal, combining traits from two different species within the Equus genus. While the mule is the more frequently produced hybrid, the reverse cross—between a male horse and a female donkey—produces a different hybrid known as a hinny. Both hybrids are valued for their endurance, strength, and generally calm disposition.
Distinguishing the Mule and the Hinny
A mule is specifically the offspring of a male donkey and a female horse, a pairing that is easier to achieve and more common in breeding programs. Conversely, a hinny results from the mating of a male horse (stallion) with a female donkey (jenny or jennet). This reciprocal cross is less common, partly because female donkeys are less likely to conceive from a horse stallion.
Mules often inherit a body shape similar to a horse but display a donkey’s characteristic long ears and coarse mane. They typically make a sound that is a mix of a bray and a whinny. Hinnies tend to be smaller and more horse-like in appearance, often having a finer build and a head that closely resembles a horse. The hinny’s ears are shorter than a mule’s, sometimes leading them to be mistaken for small horses. Mules are larger, stronger, and better suited for heavy work, while hinnies are often considered easier to handle and more docile.
The Genetic Basis for Hybridization
The ability of a horse and a donkey to produce offspring stems from their close evolutionary relationship, despite being classified as separate species. A difference in their genetic makeup influences the hybrid’s biology. A horse (Equus caballus) possesses 64 chromosomes in its body cells, while a donkey (Equus asinus) has 62 chromosomes.
When the donkey and horse reproduce, each parent contributes exactly half of its chromosomes to the offspring’s first cell. The horse parent’s egg cell contains 32 chromosomes, and the donkey parent’s sperm cell contains 31 chromosomes. Whether the offspring is a mule or a hinny, the resulting hybrid cell contains a total of 63 chromosomes. This odd number is the direct consequence of combining the haploid sets of two species with different total chromosome counts. The presence of this uneven number of chromosomes dictates the biological fate of the mule or hinny.
The Biological Reason for Sterility
The 63-chromosome count is the direct cause of the hybrid’s near-universal sterility. This reproductive failure occurs during meiosis, the cell division process that creates sex cells (gametes). Meiosis requires that chromosomes pair up precisely with their homologous partner from the opposite parent.
In a mule or hinny, the 63 chromosomes cannot be divided into 31 pairs and one unpaired chromosome, because the 31 chromosomes from one parent are not perfectly homologous with the 32 chromosomes from the other. The structural differences between the horse and donkey chromosomes, in addition to the uneven number, prevent the necessary accurate alignment and pairing. This failure disrupts the meiotic process, leading to the formation of non-viable gametes or a complete failure to produce them. The inability to create functional sperm or eggs is the biological reason why both mules and hinnies are reproductively sterile.