What advantages, if any, might be attached to maintaining at low frequencies, genes that are generally harm?

Answer 1

The gene for sickle cell anemia is kept active in populations where malaria is common, since the heterozygous (Rr) genotype provides some resistance against malaria, while the homozygous recessive (rr) is harmful to the individual. If the frequency of the gene is relatively low, the population can have some protection against malaria, while losing a few members to anemia.