Question

Despite the devastating effects of sickle-cell anaemia, why does the mutated allele responsible for this disease remain prevalent in some regions of the world? In your answer, explain the roles of relative fitness and natural selection.

Answer 1

Sickle Cell anemia is a disease inherited from parents through autosomal recessive pattern.

So to actually have the condition the offspring gets a recessive allele copy from both parents.

The mutation for this disease occurs in chromosome 11 so it is not sex linked disease.

This is the chromosome responsible for Haemoglobin production, which is responsible for binding with Oxygen.

The gene responsible for this HBB. The alleles for this are HbA and HbS.

HbS is the recessive allele responsible for the sickle cell trait.

So if both the parent has HbS and it is passed on to the offspring then the condition occurs, SCD (Sickle cell disease).

We can see this through the following Punnet square where only one parent is the carrier for sickle cell trait.

Father - HbA HbA (Normal) Mother HbA HbS (carries sickle cell trait)

	HbA	HbA
HbA	HbAA	HbAA
HbS	HbSA	HbSA

Th colored box in the Punnett square shows that the next generation will have 50%chance of carrying the sickel cell trait and can passs on the disease if they marry a person who also carries the sickle cell trait.

Why does the mutated allele responsible for this disease remain prevalent in some regions of the world?

The reason for mutated allele remaining prevalent in certain regions of the world:

a. The person carrying the sickle cell trait marrying a person carrying the defect as they do not show the symptoms.

b. Since people carrying the SCT (sickle cell trait) do not show symptoms they are diagnosed as carriers of the disease.

c. Malaria being predominant in the African regions there was natural selection with people with SCD or SCT being immune to the disease.

d. The reason for immunity in SCT is that, when these carriers blood cells were infected with the malarial parasite their blood cells (collapses) forms a sickle shape which is then removed by the spleen along with the parasite. So people with SCT survived while those without the disease succumbed to malaria.

e. In sickle cell anemia the relative fitness also can be explained as people carrying the trait for sickle cell survive for a longer period into their reproductive age compared to their normal counterparts in the malaria prone environment.