Question

why is one copy of sickle crll anemia gene beneficial while two copied are detrimental?

Answer 1

Sickle cell anemia is the result of a point mutation, a change in just one nucleotide in the gene for hemoglobin. This mutation causes the hemoglobin in red blood cells to distort to a sickle shape when deoxygenated. The sickle-shaped blood cells clog in the capillaries, cutting off circulation.

The cause of sickle cell anemia was attributed unequivocally to a single base substitution in the DNA sequence of the gene encoding the beta chain of hemoglobin, the protein that carries oxygen in red blood cells.

Having two copies of the mutated genes cause sickle cell anemia, but having just one copy does not, and can actually protect against malaria an example of how mutations are sometimes beneficial.

Sickle cell trait provides a survival advantage, against malaria fatality, over people with normal hemoglobin in regions where malaria is endemic.

In case of Sickel cell people,the RBC, which appears normal at the time of invasion, once infected undergoes sickling (probably as a result of deoxygenation and lowering pH caused by the parasite), and thus it falls easy prey to macrophages in the spleen, in other organs and even in the peripheral blood. Phagocytosis of a parasitized red cells clearly interrupts the schizogonic cycle and thus the parasitaemia can be kept under control.

Apart from this, protective effect is heme oxygenase-1 (HO-1), an enzyme whose expression is strongly induced by sickle hemoglobin. This enzyme, that produces the gas carbon monoxide, to confer protection against cerebral malaria.