In humans, sickle-cell anemia is caused by an allele that results in abnormal hemoglobin that sickles red blood cells in certain situations. individuals who are heterozygous for normal and sickle-cell alleles are protected from malaria; homozygous normal has no protection and homozygous sickle-cell has symptoms such as low tolerance for exertion and a shortened life span. this is an example of

This is an example of balanced polymorphism.  This occurs when the heterozygous individuals persist in a population because homozygous individuals are less able to survive compared to the heterozygous individuals. In the case of sickle-cell anemia, individuals with two copies of the β-globin sickle-cell allele develop sickle-cell disease and generally don't survive into adulthood, whereas individuals with one copy of the β-globin sickle-cell allele and one β-globin wild-type allele have functional red blood cells that are also resistant to malaria. We can expect this example of balanced polymorphism to occur in areas prone to malaria.