Introduction
Sickle cell anemia is a form of anemia, most commonly see in people of African descent. The physiological defect observed in sickle cell anemia is the tendency for red blood corpuscles to take up a sickle shape when conditions of reduced oxygen tension occur. A human gene mutation is the cause of sickle cell anemia (Korf 2000, p.17).
Sickle Cell Anemia
Mutations that occur can be of two types, which are point mutations that are caused by nucleotide substitutions and mutations that occur as a result of larger strings of nucleotides. Sickle cell anemia occurs as a result of a point mutation in which missense mutation leads to the amino acid substitution that has an influence on protein function. In sickle cell anemia, a change of A to T occurs in the B-globin that converts glutamate residue to valine (Primrose and Twyman 2004, p.91).
This single point mutation converts normal hemoglobin HbA to HbS: B19Gln → Val. HbS/HbA heterozygotes are affected by a milder form of sickle cell anemia than in the case of HbS/HbS homozygotes. The surface of the red blood cell develops a stocky hydrophobic patch as a result of this mutation, which is normally interrupted by the polar Gln side chain. This causes the deoxy form of hemoglobin to form polymers that precipitate within the red blood cell, going on to result in deficits in functioning of the red blood cell and sickle cell disease (Lesk 2004, p.258).
Sickle cell disease is an example of an autosomal recessive trait. Individuals affected by sickle cell anemia possess two hemoglobin S (HbS/HbS) genes by inheriting one from each parent. It is quite possible for an individual to inherit sickle cell anemia from parents who show no sign of the disease. They are heterozygotes or carriers of the mutant beta-globin gene (HbA/HbS). In both parents with sickle cell disease, for their children to develop the disease, each child has to receive the mutant allele. When one of the parents is a normal homozygous person (HbA/HbA), then the children will be unaffected heterozygotes. When a person with sickle cell anemia marries a heterozygote, then there is only a fifty percent chance of the children developing the disease. Thus for the disease to really manifest itself, both the parents need to be affected by the disease. This is typical of an autosomal recessive disease, and it is this reason that has caused the persistence of the sickle cell trait, as there is no certain continuity of the disease to the succeeding generation (Connor and Ferguson-Smith 1997, pp. 71-73).
Individuals with sickle cell anemia (HbS/HbS) and heterozygotes (HbS/HbA) have been found to be more resistant to malaria. The exact mechanisms that have led to this resistance are not clear. However, what is clear is that the HbS mutation present in heterozygotes is responsible for the resistance to malaria. This does not mean that they are totally protected from malaria; it is that they demonstrate resistance to the disease when compared to a homozygous person (HbA/HbA) (Ferguson-Smith 1997, p. 151).
Conclusion
The mutation that has led to sickle cell anemia is both a curse and a boon. It is a curse for those who have the disease (HbS/HbS) because of the consequences of the disease. On the other hand, it is a boon to the heterozygotes (HbS/HbA), as it offers resistance from the dreaded disease of malaria.
References
Connor, Michael & Ferguson-Smith, Malcolm.1997. MEDICAL GENETICS. Fifth Edition. Oxford: Blackwell Science.
Lesk, Arthur, M. 2004. INTRODUCTION TO PROTEIN SCIENCE: Architecture, Function, and Genomics. Oxford: Oxford University Press.
Korf, Bruce. R. 2000. Human Genetics: A Problem Based Approach. Second Edition. Massachusetts: Blackwell Science.
Primrose, Sandy, B. & Twyman, Richard, M. 2004. Genomics: Applications in Human Biology. Oxford: Blackwell Science.