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Heterosis, better known as hybrid vigor, also called outbreeding enhancement, is an observable phenomenon in which a hybrid plant or animal may exhibit greater strength, health or faster rate of growth than its parents. The term often causes controversy, particularly in terms of domestic animals, because it is sometimes believed that all crossbred plants or animals are better than their parents; this is untrue. Rather, when a hybrid is seen to be superior to its parents, this is known as hybrid vigor.

Offspring inherit characteristics from their parents, usually a mixture of what we humans see as 'good' and 'bad' traits. Inbreeding concentrates and reinforces genes, good and bad, over time. When offspring pick up only undesirable traits, it is usually as a result of inbreeding. This is known as inbreeding depression, and outcrossing, or outbreeding introduces new genetic elements and can result in heterosis. However, this is not guaranteed; it may also happen that a hybrid inherits and exhibits the worst qualities of each of its parents, or is inferior to each; it may even be unfit for survival. This is called outbreeding depression. Sterility is a common manifestation of this, as in a mule, a hybrid of a donkey and a horse that is always incapable of reproduction.