|[b]Mutations and Malaria[/b]
A mutation is responsible for a different sequence of amino acids in the globin chain of á and â subunits in haemoglobin. The mutation in the genes controlling the subunit production affects the globin chain in its structure and therefore changes its behaviour in the human body.
People which have sickle-cell haemoglobin differ from normal haemoglobin by only one amino acid. The abnormal causes in the body are as follows:
- Sickle-cell haemoglobin is less soluble in low partial pressure of oxygen (which is in the veins).
- Haemoglobin molecules stick together and form long fibres. This alters the shape of red blood cells.
Because the shape of red blood cells is affected the body destroys these abnormal cells. Affected persons develop severe anaemia. As sickle cells clump together they may block smaller blood vessels, and overall, the supply of oxygen to the organs of the body is reduced. People homozygous for that disease are likely to die in early childhood!
People which are heterozygous only have a single copy of the defective allele. The result is a production of SOME sickle-cell and some normal haemoglobin. Patients only show a few symptoms of sickle-cell anaemia and have a normal behaviour in normal conditions. Some red blood cells only become sickle shaped under conditions such as vigorous exercises or high altitudes.
The advantage of people possessing the heterozygous allele is that they cannot get malaria. The malaria parasite causes red blood cells to become sickle shaped and these sickle shaped cells are immediately recognized by the body’s own immune system. The destruction of the cell follows and the parasites it contains are unable to complete their life cycle!
People with a heterozygous allele are rarely found in countries which lack the malaria parasite. But in countries where the parasite is found in large numbers the incidence of people being heterozygous for the allele responsible for sickle-cell anaemia is increased by a huge amount. This is explained by natural selection which produces a balanced polymorphism.