(a). Balanced polymoprhism, by selecting for the heterozygote, would explain why sickle-cell hemoglobin remains more common in African populations (where the advantage of greater resistance to malaria favors the heterozygous individuals) than in North America (where malaria is not a problem and hence not a selective agent in the environment). The other observations are important aspects of the epidemiology of sickle-cell anemia, but are not explained by the concept of balanced polymorphism.