Question

The sickle cell anemia allele is very common for a harmful mutation. Why? (in-depth answer pls)

Answer 1

Sickle cell disease is a group of disorders that affects hemoglobin, which is the molecule in red blood cells that delivers oxygen to cells throughout the body. It is a recessive genetic disease, which means that both copies of the gene must contain the mutation for a person to have sickle cell anaemia.

Sickle cell disease is caused by a mutation in the hemoglobin Beta gene found on chromosome 11. Hemoglobin transports oxygen from the lungs to other parts of the body. Red blood cells with normal hemoglobin (hemoglobin-A) are smooth and round and glide through blood vessels. A mutation as simple as a single base change in a DNA sequence can have dramatic effects on phenotype.

A baby born with sickle cell disease inherits a gene for the disorder from both parents. When both parents have the genetic defect, there's a 25 percent chance that each child will be born with sickle cell disease. Every person has two copies of the hemoglobin gene. Usually, both genes make a normal hemoglobin protein. When someone inherits two mutant copies of the hemoglobin gene, the abnormal form of the hemoglobin protein causes the red blood cells to lose oxygen and warp into a sickle shape during periods of high activity. Their shape causes these red blood cells to pile up, causing blockages and damaging vital organs and tissue.

Sickle cells are destroyed rapidly in the bodies of people with the disease, causing anemia. This anemia is what gives the disease its commonly known name - sickle cell anemia.

The sickle cells also block the flow of blood through vessels, resulting in lung tissue damage that causes acute chest syndrome, pain episodes, stroke and priapism. It also causes damage to the spleen, kidneys and liver. The damage to the spleen makes patients - especially young children - easily overwhelmed by bacterial infections.

Heterozygotes (AS) with the sickle cell allele are resistant to malaria. The red blood cells, containing some abnormal hemoglobin, tend to sickle when they are infected by the malaria parasite. Those infected cells flow through the spleen, which culls them out because of their sickle shape and the parasite is eliminated along with them. Therefore, they are more likely to survive and reproduce. This keeps the sickle cell allele in the gene pool.