How is the sickle-cell allele maintained through natural selection?

Answer 1

Heterozygous induviduals pass the dominant and recessive alleles to offspring

Answer 2

Sickle cell anemia alleles are maintained in human populations because of a phenomenon called heterozygote advantage. Though it is true that sickle cell anemia is a very debilitating disease and will lower an individual's fitness tremendously, it also confers immunity to malaria, a very prevalent and deadly disease in many parts of the world. This is because the protist parasite that causes malaria normally spends a part of its life cycle inside the host's red blood cells, and the strange shape and excessive aggregation of red blood cells in a person with sickle cell anemia is too hostile an environment for the parasite to colonize. However, even though sickle cell anemics can survive a malaria infection, it still shortens the individual's life span considerably and therefore this immunity may not actually increase the individual's fitness. Now what is important to realize is that heterozygotes, i.e. individuals who are heterozygous for the sickle cell anemia locus are also immune towards malaria without the deleterious effect of full-blown sickle cell anemia. This way, a heterozygote has an advantage because he neither suffers the ill effects of sickle cell anemia nor is susceptible to malaria infection. This confers increased fitness to heterozygotes, and when they mate there is at least a 50% probability that the offspring will also be heterozygous. This way the sickle cell anemia allele persists in human populations.

Answer 3

Heterozygous induviduals pass the dominant and recessive alleles to offspring

Answer 4

Individuals with two recessive alleles have very high rates of reproduction.