In certain areas of the world where there's a high incidence of malaria, there's also a high incidence of sickle-cell anemia. Sickle-cell anemia results from a mutation in the gene that codes for hemoglobin. Malaria is an infectious disease carried by mosquitoes. Both malaria and sickle-cell anemia can be fatal. If an individual is homozygous for normal hemoglobin, they're more prone to malaria. If an individual is homozygous for sickle-cell hemoglobin, they'll develop sickle-cell anemia. However, if an individual is heterozygous and possesses one allele for normal hemoglobin and one allele for sickle-cell hemoglobin, they're less prone to malaria and won't develop sickle-cell disease. What description applies to this example?