#133 Sickle cell anaemia and its incidence to that of malaria

Normal and sickle red blood cells.
Credit: Wellcome Trust, UK

Sickle cell anaemia is caused by a mutation in the blood pigment haemoglobin. When the faulty haemoglobin is present in a red blood cell, it causes the cell to deform and become sickle-shaped, especially when oxygen levels in the blood become low.

In this state the sickled red blood cells are less efficient at transporting oxygen and more likely to become stuck in a capillary, preventing blood flow.

The faulty allele is dominated by the allele for normal haemoglobin, but still has some effect in a heterozygous genotype.

The possible genotypes are:

H^NH^N normal haemoglobin, no anaemia
H^NHⁿ some abnormal haemoglobin, sickle cells trait (not life-threatening)
HⁿHⁿ abnormal haemoglobin, sickle cells anaemia (life-threatening)

Malaria is a life-threatening disease caused by a parasite that invades red blood cells. The parasite is carried by some species of mosquito.

A person who is heterozygous (H^NHⁿ ) for sickle cell anaemia has protection from malaria, because the malaria parasite is unable to invade an reproduce in the sickle cells.
A person who is homozygous for sickle cell anaemia (HⁿHⁿ) also has protection, but is at high risk of dying form sickle cell anaemia.
A person with normal haemoglobin (H^NH^N) in a malarial country is at high risk of contracting malaria.

When the distributions of malaria and sickle cell anaemia are shown on a map of the work, it is found that the two coincide in tropical areas because of the selective advantage of the Hⁿ allele in providing protection against malaria.

Video: Sickle cell disease