Variation

When we examine the fitness values of the 3 genotypes in the population an interesting pattern emerges. Individuals with 2 copies of the sickle cell allele have low fitness because they die from anemia, even though they are resistant to malaria. Individuals with normal hemoglobin have reduced fitness because they are susceptible to malaria. Individuals who are heterozygotes have the highest fitness because they do not suffer from anemia or malaria. The reason heterozygotes are resistant to malaria is because roughly half of their hemoglobin is of the sickle cell type. While there is sufficient normal hemoglobin to prevent anemia, the malaria parasite cannot survive in red blood cells with a high amount of sickle type hemoglobin. Cases such as this, where the heterozygote is the most fit genotype, are called heterozygote superiority or balanced polymorphism. In these situations, allelic variants are maintained in a population.


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