Heterozygote Advantage

A heterozygote advantage is when the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. This is due to a single locus known as overdominance, which is a condition where the phenotype of the heterozygote lies outside the phenotypical range of both homozygote parents, and heterozygous individuals have a higher fitness than homozygous individuals (1,2). The advantages and disadvantages are often complicated, because more than one gene may influence a given trait. Major genes usually have multiple effects which can simultaneously convey separate advantageous traits and disadvantageous traits, while both homozygotes convey a disadvantage.

Heterozygote advantage is a major mechanism for "hybrid vigor" which is the improved or increased function of any biological quality in a hybrid offspring. A majority of cases- especially in plants- are due to dominance and the masking of deleterious recessive alleles by wild-type alleles. But, there were also findings of overdominance, especially in rice. Recently, research has established that there is an epigenetic contribution to heterozygote advantage (3,4).

Sources:
 * 1)  Charlesworth D, Willis JH (November 2009). "The genetics of inbreeding depression".Nat. Rev. Genet. 10 (11): 783–96. doi:10.1038/nrg2664. PMID 19834483.
 * 2) Carr DE, Dudash MR (June 2003). "Recent approaches into the genetic basis of inbreeding depression in plants". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 358(1434): 1071–84. doi:10.1098/rstb.2003.1295. PMC 1693197. PMID 12831473.
 * 3) Chen ZJ (February 2010). "Molecular mechanisms of polyploidy and hybrid vigor".Trends Plant Sci. 15 (2): 57–71. doi:10.1016/j.tplants.2009.12.003. PMC 2821985.PMID 20080432.
 * 4) Baranwal VK, Mikkilineni V, Zehr UB, Tyagi AK, Kapoor S (November 2012)."Heterosis: emerging ideas about hybrid vigour". J. Exp. Bot. 63 (18): 6309–14.doi:10.1093/jxb/ers291. PMID 23095992.