Heterosis: The Case for Single‐Gene Overdominance

Although the phenomenon of hybrid vigor/heterosis has been observed by naturalists and harnessed by breeders for centuries, the molecular basis for the transgressive phenotypes observed in hybrid offspring has remained elusive. Here, we first make a distinction between hybrid vigor (natural populations) and heterosis (domesticated breeding populations) and then discuss the theories of heterosis in an agricultural context, with a particular focus on the most controversial theory, single-gene overdominance. Documented cases of single-gene overdominance driving heterosis are rare, yet striking examples have been described in diverse species including yeast, plants, and animals. We argue that gene dosage arising from allelic heterozygosity in as little as a single gene can drive significant transgressive changes on growth and development, that such dosage effects are likely widespread in nature, and that special cases can be harnessed to drive heterosis in crops.