Question

sickle cell distribution compared to malaria distribution

Answer 1

Sickel cell anemia exhibits heterozygote advantage.

Sickel cell mutation causes shrinkage of RBC which reduces the efficiency of blood to transport gases to the tissues. This leads to anemia. However, the mutation is under positive selection even though it is a deleterious/harmful allele. This is because of the selection pressure exerted by malarial parasite. Malaria is caused by Plasmodium which dwells in the RBC of the infected people. Sickle cell RBC can not sustain the rapid growth of Plasmodium as their carrying capacity of oxygen is less. So, It can infect and sustain in homozygous HbS/HbS individuals.

Individuals homozygous for HbS allele are destined to develop anemia as their RBC can not sustain the gaseous transport required for the body cells.

Heterozygotes contain a WT allele and an HbS allele. These heterozygotes can sustain their body oxygen requirement as they contain one copy of the WT gene. However, Plasmodium is very sensitive to low oxygen content in RBC. So, it can not sustain in heterozygous RBC. Hence these people show a selective advantage over WT homozygous people (WT homozygous people are susceptible to malaria).

Hence, the HbS allele is maintained in populations only where the selection pressure (Plasmodia infection) is prevalent.

Ex: Angola, Benin, and Burundi