04 April 2014

#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:
  • HNHN normal haemoglobin, no anaemia
  • HNHn 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 (HNHn ) 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 (HNHN) 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 

Video: What is Sickle cell anaemia 

Video: Evolution of sickle cell malaria

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