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Heterosis is increased strength of different characteristics in hybrids; the possibility to obtain a "better" individual by combining the virtues of its parents.
This is commonly known as hybrid vigor. It 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.
Reverse hybrid vigor
It may also happen that a hybrid inherits and exhibits the worst qualities of each of its parents, or is inferior to each. 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.
Another example of "reverse hybrid vigor" is the liger, a hybrid of male lion and a female tiger. Ligers are generally sterile. More importantly, ligers lack a growth-inhibiting gene, meaning that they do not stop growing until they become so large they die from heart failure or other complications resulting from uninhibited growth. Tigons, resulting from a mating of a male tiger and a female lion, have the opposite problem. These hybrids receive two versions of the growth-inhibiting gene from their parents, making them much smaller when fully grown.
The adjective "heterotic" is commonly used in high-energy physics - the Censored page theory is the leading candidate to describe all particles and forces in nature.