Balanced polymorphism refers to the maintenance of two or more genetically distinct forms, or alleles, within a population at relatively stable frequencies over many generations. In human genetics, it is an important concept because it explains why certain alleles that may have harmful effects are nevertheless retained in human populations. The persistence of such genetic variation generally results from natural selection acting in a way that gives some advantage to individuals carrying particular combinations of alleles. One of the most important mechanisms responsible for balanced polymorphism is heterozygote advantage, also called overdominance. In this situation, individuals who are heterozygous for a particular gene have greater survival or reproductive fitness than individuals who are homozygous for either allele. Consequently, both alleles are maintained in the population. The classic example in human genetics is the relationship between the sickle-cell allele and malaria. The sickle-cell allele, represented as HbS, can cause sickle-cell disease when present in the homozygous state (HbS/HbS). Individuals