Overdominance
From Freepedia
Overdominance describes a genetic relationship between two alleles when the phenotype of the heterozygote is "greater" than that of either homozygote.
Overdominance, unlike heterozygote advantage, does not necessarily involve genetic selection for increased fitness of the heterozygote. Rather, overdominance may refer strictly to a biochemical interaction between either gene products or regulatory elements (promoters) of the two alleles resulting in greater gene activity.
Compare overdominance to the term heterozygote advantage, used in population genetics.
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