in the african tropics the maintenance of the sickle-cell allele in the human population can be attributed to_?

stabilizing selection

people heterozygous for HgbS alelle are carriers of sickling hemoglobin. People homozygous for HgbS allele suffer from sickle cell anemia. Normal adults are HbA. In African tropics the mainteance of the sickle cell allele is due to Malaria. The malaria virus is not able to live in the red blood cell with the mutated hemaglobin because those cells are fragile, smaller, and have a shorter life span. This benefits the person with sickle cell trait because they will not get malaria. because in areas where malaria is common, there is a survival value in carrying only a single sickle-cell gene (sickle cell trait). Those with only one of the two alleles of the sickle-cell disease are more resistant to malaria, since the infestation of the malaria plasmodium is halted by the sickling of the cells which it infests. So the selections favors those that are heterozygous for the HgbS alelle, the intermediate trait, thus it's stabilizing selection.

Ah, a properly stated case. Malaria lives in pink blood cells (RBCs). notwithstanding, persons who carry the sickle cellular trait - are heterozygous for it - have 50% malformed RBCs. meaning 1/2 as a lot malaria (perchance the larvae, not effectual) infect them, allowing their bodies to mount an attack on the malaria. Sickle cellular is so named because the RBCs formed from this allele seem as if a sickle - curved, hollow, as antagonistic to round.

I think it's stabilizing selection.

Stabilizing selection- A process of natural selection that tends to favor genotypic combinations that produce an intermediate phenotype; selection against the extremes....

Sounds like the sickle-cell situation to me. Either extreme leads to a pretty good likelihood of death, and even though the intermediate route leads to circulatory problems, at least you remain alive.