The recessive allele that causes sickle cell anemia is harmful to homozygous individuals. what maintains the presence of this allele in a population's gene pool?

The one responsible for maintain the presence of this allele in the population’s gene pool is the heterozygote advantage. It is because the advantage of having this for a sickle cell disease is that if a person has a strong heterozygote advantage and that he or she has a copy of the allele that cause the sickle cell disease, the person will likely be resistant to the disease because of the heterozygote advantage in which the heterozygote genotype has a higher relative fitness making it resistant to the disease.

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if the hares very faster than they are now, and thus the lynx's hunting success rate drops because it will be harder to catch them, the rate that will change will be the pray-rate. the pray-rate will change because more hares will be able to survive, thus more individuals for reproduction, so the end result will be increasing population of hares.

the predator-rate will remain pretty much the same because there will be more than enough pray, but the hunting success rate is stagnant.