why are people that are heterozygus for sickle cell resistant to malaria?

Question

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Answer 1

Sickle cell is a codominant trait. People with two copies of the gene for sickle cell have sickle cell disease or sickle cell anemia because low oxygen levels make their hemoglobin turn into long needle-like crystals and their red blood cells sickle. Not good.

People with no copies of the gene for sickle cell are at risk of dying from malaria in an area that has a bad malaria problem.

People who are heterozygous make both normal hemoglobin and hemoglobin that will sickle. When the malaria parasite gets into the body and does its time in the liver and gets into the red blood cells, it proceeds to reproduce inside the red blood cells. The parasite is using a lot of oxygen to do that. The resulting low oxygen levels cause the hemoglobin to sickle in the cells infected with the parasite -- those cells die, and the parasite can't continue its life cycle. Meanwhile, the person's uninfected red blood cells are fine. So having sickle cell "trait" conveys some protection against dying from malaria while it doesn't give the person enough sickle-type hemoglobin to kill them from sickle cell anemia.

Answer 2

an autosomal recessive disorder is where the mutation affects a transport protein: 50% activity is still well tolerated. Example: Sickle cell anemia. the gene of beta-globin is mutated. in homozygotes, mutant hemoglobin clumps together in red blood cells, deforming them and causing obstruction of small blood vessels. Life span of RBCs is shortened. Symptoms: anemia, pain caused by hypoxia etc. Although sickle cell anemia is a severe condition imposing significant mortality on the affected homozygotes (ss genotype), the mutant allele has not disappeared by selection, as we could expect for a highly disadvaantageous allele. This is because heterozygotes, although they carry the mutant gene and produce the mutant protein, do not suffer from the symptoms of sickle cell anemia. on the other hand, Ss heterozygotes are remarkably resistant to malaria, a major cause of death in the equatorial region. thus heterozygote advantage, a selective reproductive advantage of heterozygotes, saved the s allele from being "selected out" during human evolution