The Sickle Cell Trait: An Evolutionary Advantage?

 by Annika Bright

Sickle Cell Disease is known to be one of the most widespread genetic disorders worldwide. It is endemic in countries in Africa including Nigeria, with around 150,000 babies born with SCD annually. The fact that over 80% of patients with SCD live in central Africa, India and the Middle East establishes its link with Malaria. 

Sickle Cell Disease itself, causes the haemoglobin - used to transfer oxygen around the body for aerobic respiration to occur - to arrange itself into thin, rod-like structures within the red blood cell. This ultimately causes the red blood cell to have a modified, “sickle” shape. Although at first glance this may not appear to be detrimental, the ramifications of the disease cause multiple blockages or clots in multiple vital blood vessels which may include the carotid arteries that  provide the brain with a constant supply of oxygen. A blockage within this artery (stroke) will cause neuronal death due to a rapid reduction in cerebral blood flow, preventing the transport of oxygen. Sickle Cell Disease is caused by a defective Haemoglobin A gene which is composed of two alpha-Globin and two Beta-globin protein chains. Within the disease, there is a mutation in the HBB gene which results in the Beta-globin to have a malformed shape. Two HBB gene copies are required for the patient to appear with Sickle Cell Disease as the disease is autosomal recessive, which means that if a person only has one copy of the HBB gene, they will only be a carrier of the disease (have the sickle cell trait) rather than appear with symptoms. Being a carrier of the disease has shown to be an evolutionary advantage within areas where Malaria is prevalent. 

Malaria is caused by the Plasmodium falciparum protozoa which is carried by a female mosquito vector. The red blood cells of SCD carriers are fragile in comparison to normal red blood cells. For example, being exposed to high altitudes or extreme temperatures could induce the sickling of these cells. Therefore, by having the sickle cell trait, the Protozoa causes the red blood cell to burst and thus, cannot replicate itself - so, the disease is less likely to appear in the carriers.