Question

Use the sickle cell anemia/malaria example to explain why alleles coding for negative traits can sometimes persist in a population over time.

Your answer should include:

• what type of trait sickle cell anemia is (in terms of genetics) and which of the 2 alleles codes for sickle cell anemia?
• what the advantages and/or disadvantages of each genotype are for this trait?
• why the sickle cell allele is still present in many populations?

Use paragraph format.

Answer 1

It is the most commonly occurring haemoglobinopathy caused by an inherited structural abnormality in the globin polypeptide. Originally described in 1910 by James Herrick, the disease was the first inherited disorder shown to arise from a specific amino acid change in a protein. Since then it has been so extensively studied, biochemically and biophysically, that it has virtually become a paradigm of a molecular disease.
Molecular Defect:
The sickle cell haemoglobin (HbS) arises out of a substitution of the glutamic acid residue in position 6 of the β chain with the valine residue (β6Glu - Val). As noted earlier, this substitution can be traced to an A - T transversion in the β chain gene and results in formation of an abnormal haemoglobin of subunit composition α2βs2. Both homozygous and heterozygous forms of disease have been described. The heterozygous form is also referred to as sickle cell trait.
Sickle cell trait (also known as being a carrier) occurs when a person has one gene for sickle hemoglobin and one gene for normal hemoglobin.Approximately one in ten African-Americans carries sickle cell trait. People who are carriers generally do not have any medical problems and lead normal lives.
The sickle cell anemia trait is found on a recessive allele of the hemoglobin gene. This means that you must have two copies of the recessive allele ,one from your mother and one from your father to have the condition. People who have one dominant and one recessive copy of the allele won't have sickle cell anemia.Sickle cell anaemia results because of a point mutation.
The sickle cell allels may be maintained by gene flow, that is why they are still present in many population.
On the contrary, individuals who are carriers for the sickle cell disease (with one sickle gene and one normal hemoglobin gene, also known as sickle cell trait) have some protective advantage against malaria. As a result, the frequencies of sickle cell carriers are high in malaria-endemic areas.