Malaria and Sickle cell anemia In USA we do not have Malaria because the protist
ID: 201611 • Letter: M
Question
Malaria and Sickle cell anemia In USA we do not have Malaria because the protist causing the disease (Plasmodium) is not present However, there are people that carry the allele that causes sickle cell anemia, in the heterozygous condition (AS) Most of these people do not suffer greatly from the disease, they are simply heterozygous for the condition One would expect that the allele for sickle anemia (S) would disappear from the population in USA since there is no heterozygote advantage and homozygous for S die early in life However, this is unlikely to happen Why? I can think of at least two reasons but more are likely You get full credit it you can successfuly articulate one reason lExplanation / Answer
When a person carries two copies of an allele it is disadvantageous, but when carrying only one copy it is advantageous, as the process of natural selection does not remove the allele from the population. the benefit is awarded in its heterozygous state which keeps the allele around. For example, we can say that the allele that causes sickle cell anaemia is pernicious if you carry two copies But if you only carry one copy of allele and you live in a place where malaria is common, the allele is profitable because it presents resistance to malaria but here since in the USA we do not have malaria there is not heterozygote advantage. Sometimes it is unlikely to happen due to few reasons like:
The heterozygous condition of allele might be maintained by mutation
The mutation that produces the pernicious allele may keep on rising in the population, even if natural selection removes it out.
The heterozygous condition of allele might be maintained by gene flow
The allele can be common, and not pernicious, in a closed environment. If gene flow from the nearby population is common, we will observe the deleterious allele in the population of interest. For example, in places like the USA where malaria is not a problem, the gene which causes sickle cell anaemia is of no use. Nevertheless, in many parts of the world, the gene that causes sickle cell anaemia is more common as it confers resistance to malaria(single copy). due to Human migration, this gene can be found in populations all over the world.
Natural selection may not have had time to remove them yet
The direction of selection changes as the environment changes for example if an allele was advantageous or vague ten generations ago may be harmful today. So it can be true that some of the harmful alleles that we observe in natural populations are going out, but the process of natural selection has not yet effectively removed them completely.
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