Malaria is a widespread endemic disease which contributes significantly to infant mortality. Genetic traits which protect against death by malaria or confer higher fertility on their carriers in malarial environments would have a distinct biologic advantage. As long as malaria remains, such traits could be expected to increase in frequency from generation to generation, so that ultimately the entire population carries the protective gene or genes. The resistance of most West Africans and of their nonimmune descendents in the United States against infections with Plasmodium vivax suggests the presence of such genes in these populations. Their nature and mode of action remains unknown. If a genetic trait protects against malaria but also has harmful effects and causes illness or diminished fertility, an equilibrium gene frequency will be reached at which the beneficial and harmful effects of such a gene balance each other.
Departments of Medicine and Genetics, University of Washington, Seattle, Washington.