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The Mule Hybrid

Author: Renee Cao

Editor: Cynthia Zhang

Artist: Tiffany Gao

When two different species, breeds, or varieties crossbreed through sexual reproduction, the result is a hybrid. The oldest and most common hybrid is the mule, which has been around for more than 5,000 years. Before camels were introduced to Egypt, mules were used as beasts of burden. Four thousand years ago, mules were considered treasures in China’s western and northern regions and in ancient Ethiopia. So what exactly is a mule? It is the offspring of a male donkey and a female horse. The crossbreeding of a female donkey and a male horse is possible, and the offspring would be called a hinny; however, hinnies have lower rates of conception; thus, they are less common. Since hinnies are a lot smaller, slower, more stubborn, and unpredictable, mules are also preferred over hinnies.

Hybrid vigor happens when an offspring’s traits are enhanced as a result of genetic mixing. Mules have been produced due to their many advantages—their features are a combination of the best characteristics of donkeys and horses. First, mules have the appearance (head-wise) and endurance of donkeys but are bigger, stronger, and more intelligent. Second, mules have similar heights and body sizes as horses but have more stamina and require less food. Third, their hooves are tougher than horses, making them the better option to carry things over rough terrain. Finally, their skin is less sensitive than horses and donkeys, which appeal to owners who work under harsher weather and stronger sunlight.

Even though mules outweigh donkeys and horses in various aspects, one drawback is their infertility—they can not produce any offspring. The only way to create a mule is by crossbreeding a horse and donkey. Most hybrid animals are sterile because the movement of genes between species is restricted. Mules, specifically, are infertile due to the difference in chromosomes between their progenitors. A horse has 64 chromosomes in each cell, a donkey has 62 chromosomes in each cell, while a mule/hinny has 63 chromosomes in each cell. Chromosomes carry an organism’s genetic content and the instructions needed for cell division. Chromosome abnormalities or differences in the number or structure of chromosomes can lead to disease, cancer, and development problems. In sexual reproduction, chromosomes come in pairs, as each set comes from each of the two parents. Since 63 is an odd number, there is an extra chromosome. The extra chromosome is a problem in meiosis, the cell division that produces gametes, or egg and sperm cells. As the extra chromosome can’t create a homologous pair, mules are unable to produce gametes that allow them to reproduce.



“Hinny vs. Mule: 11 Fun Facts.” Helpful Horse Hints, 14 Aug. 2019, h

“History of the Mule.” American Mule Museum,

Mules (Donkey × Horse) - Mammalian Hybrids - Online Biology Dictionary.

“What Is A Mule? 13 Things You Didn’t Know.” SPANA, 18 Jan. 2019,

Wilford, John Noble. “Camels Had No Business in Genesis.” The New York Times, 10 Feb. 2014.

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