You may wonder where mules fit into all of this equine stuff. Mules are actually a hybrid animal, combined by crossing a horse with an animal from the ass family. When a mare is bred to a jack, the offspring is called a mule. You can also breed to get a hinny, the product of a female donkey and a stallion. There are some interesting scientific principles behind these hybrid animals. You see, even though they have a distinct male or female gender, all mules (and hinnies) are sterile, meaning they cannot produce offspring. This is because of some very specific things that happen in the creation of their DNA. During the process of meiosis, sperm and egg cells are created. These cells, also called sex cells, have half the regular number of chromosomes for that species. For example, horses have 64 chromosomes, meaning that a horse egg or sperm cell only has 32 chromosomes. In donkeys, there are 62 total chromosomes, leading to sperm and egg cells that have 31 chromosomes. When donkeys and horses mate, the result is a zygote (newly fertilized egg) that has 63 chromosomes. Because the DNA structure of horses and donkeys is so closely related, this zygote grows into a complete organism with 63 chromosomes. Mules function well until it comes time for them to create sex cells of their own. In meiosis, there is no way to split an uneven number of chromosomes, leaving mules and hinnies unable to create viable sperm or egg cells. Mules: Hybrid Animals Connection STEM Jack 62 Chromosomes (2n = 62) Egg n = 32 Sperm n = 31 Mule 63 Chromosomes (2n = 63) Mare 64 Chromosomes (2n = 64) 556 Principles of Agriculture, Food, and Natural Resources Copyright Goodheart-Willcox Co., Inc.