a characteristic exhibited when the mean hybrid phenotype exceeds the range of the parents —note 1. statistically, heterosis is the increase in a trait over the mean value for that trait in the parents —note 2. heterosis is usually applied to traits such as size or general vigor —see F₁, heterozygous, hybrid, inbred line, inbreeding, phenotype, transgressive segregation —synonym hybrid vigorThis definition last updated 12/09/2008