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The question of why certain progeny of crosses or hybrids between unrelated individual plants and animals show greater vigor, productivity, disease resistance, viability and fertility, or Darwinian fitness as opposed to the progeny of close relatives has fascinated humans for millennia.“
[Source below]
For reasons yet to be identified, genetic crosses between closely-related plants (in order to produce hybrids) tend to result in new plants which are stronger in many respects than either of the two 'parent' plants.
The phenomenon has been termed heterosis - more commonly known to plant breeders and gardeners as 'hybrid vigour'.
There are two main competing theories which attempt to explain the phenomenon. The Dominance Hypothesis and the Overdominance Hypothesis (see paper reference below for details). More recently, there are suggestions that it may also have epigenetic (outside of the genes) components.
Despite countless numbers of studies on the manifestation of heterosis in diverse organisms gathered over decades, simple biological principles could remain elusive.
[Source below]
Quotes are from PLoS Biol 17(4): e3000215 
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