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Heterosis, better known as hybrid vigor, also called outbreeding enhancement, is an observable phenomenon in which hybrid plants or animals may exhibit greater yields, health or faster rate of growth than their inbred parents. A common misconception, particularly in terms of domestic animals, is that all crossbred plants or animals are better than their parents; this is untrue. Rather, when a particular 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 different versions of genes, known as alleles, over time. The fixation of alleles leading to undesirable traits are the most noticeable result of inbreeding. This is known as inbreeding depression, and outcrossing, or outbreeding introduce new allelic variation 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.