What is meant by heterozygote advantage?

Heterozygote advantage, also known as balancing selection, arises when there is a survival (selective) advantage for hybrids (heterozygotes) as compared to that of either homozygous recessive genotype or homozygous dominant genotype. Some diseases are lethal only in the homozygous state. If an allele is lethal, then why would it not disappear from a population totally? In the example of cystic fibrosis, heterozygotes may be more resistant to life-threatening dehydration caused by diseases such as cholera, which is accompanied by massive diarrhea. In cholera, caused by the lethal bacterium Vibrio cholera, the bacteria produce a toxin that causes considerable loss of water, dehydration, and death within a short time. Another example of heterozygote advantage is the high frequency of heterozygotes for sickle cell anemia in Africa (and increased resistance to malaria).

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