In 2012, Ryan Clark of the Pittsburgh Steelers was forced by his coach to sit out of a playoff game in Denver due to health concerns. Clark is a heterozygote for sickle cell anemia.
Sickle cell anemia is a recessive condition that results in misshapen red blood cells due to a mutation in the hemoglobin gene. The condition greatly reduces the ability of red blood cells to carry oxygen.
Heterozygotes generally do not have symptoms of the condition, but will sometimes have problems when dehydrated or at high altitudes. In addition to reduced oxygen carrying ability, the sickle cell allele has another trait; resistance to malaria.
Some areas of the world have a sickle cell allele frequency of up to nearly 20%. What would you expect to find in areas of the world with high sickle cell frequencies?