Question

The population in South Africa is more prone to sickle cell anemia (genotype "ss"). People with genotype "ss" survive only up to 14-15 years of age. People with "SS" are prone to malaria because the parasite resides inside their RBCs. People with genotype "Ss" are resistant to malaria because the parasite cannot reside inside their RBCs, and they can live a normal life (without sickle cell anemia).What kind of evolutionary force must be operative in South Africa?
A.
Stabilizing selection
B.
Directional selection
C.
Disruptive selection

Answer 1

Answer:

(a) stabilizing selection

Explanation:

Sickle-cell anaemia is a recessive disorder caused by the presence of two recessive alleles "s", so the genotype is "ss". This disorder is characterized by sickle haemoglobin. In an area with malaria, heterozygous individuals "Ss" (with one dominant allele and one recessive allele) have an advantage. These individuals will have both normal and sickle haemoglobin. But pathogen that causes malaria affects only normal haemoglobin, so heterozygous individuals will have half of the haemoglobin resistant to the pathogen and those individuals are resistant to malaria.

Stabilizing selection favours heterozygotes Ss,  disruptive selection favours dominant (SS) and recessive (ss) homozygotes, while directional selection favours dominant (SS) or recessive (ss) homozygotes. Since in this example, people with genotype Ss (heterozygotes) are at an advantage, then this is an example of stabilizing selection.

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