c. balanced polymorphism
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Balanced polymorphism leads to a higher incidence of having individuals heterozygous for the trait in a population. The gene that codes for the normal form of hemoglobin is HbA, but it is codominant with a mutant form, HbS. This mutant allele causes sickle cell anemia. Individuals in areas where malaria is prevalent tend to have one copy of the normal gene and one of the sickle-cell gene. It seems that individual who are heterozygous survive better in malaria-prone areas than those who are homozygous. In heterozygous individuals, only some of the cells have a sickle shape; these abnormal cells, which also contain the parasite responsible for malaria, are destroyed by the body's immune system. Since individuals homozygous for the normal form have normal-looking red blood cells, the parasites in them are not found by the immune system.