Heterosis (sometimes outbreeding enhancement or hybrid vigour) describes a hybrid organism that is genetically superior to either of its parents due to the combined effect of its alleles. It is the opposite effect to outbreeding depression.

There are two hypotheses explaining heterosis, although they needn't be mutually exclusive. The dominance hypothesis asserts that hybrids are superior due to the presence of a dominant allele that silences the effect of (often harmful) recessive alleles. According to this hypothesis, both heterozygotes and dominant homozygotes are of equal fitness. The overdominance hypothesis states that combinations of alleles are advantageous in heterozygotes, even though one or more of the alleles that is advantageous in a heterozygote may be deleterious when in a homozygote. The overdominance hypothesis is often used to explain the persistence of deleterious alleles in the gene pool despite the action of natural selection (for instance, in the well-known case of sickle-cell anaemia and malaria prevention).

Both hypotheses attribute the poor performance of inbred lineages to the accumulation of homozygous recessive genotypes.