A condition where individuals with two different alleles for a gene have higher fitness than those with two identical alleles. This maintains genetic diversity by favoring the heterozygous genotype.
From Greek 'heteros' meaning 'different' and 'zygotes' meaning 'yoked together', combined with 'advantage' from Old French 'avantage'. The concept emerged in the 1940s when geneticists discovered that sometimes being genetically mixed is better than being genetically pure.
Sickle cell anemia is the classic example - having one sickle cell allele protects against malaria without causing the disease! It's like genetic insurance where diversity literally saves lives, which is why this 'harmful' allele remains common in malaria-prone regions.
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