If the proportion of an african population that are carriers for the sickle cell allele (hbs) is 0.142, what proportion of the population have a protective advantage against acquiring malaria? we restrict our attention to only those individuals with a protective advantage who are also not affected with sickle cell anemia.

A very simple change in the beta globin sequence and hence protein structure causes what is known as sickle haemoglobin (HbS). Carriers of HbS (who have one mutant and one normal copy of the gene) they have little difference in their blood counts and no symptoms. But they have about a 30% reduction in susceptibility to malaria; which is quite a  heavy protection.

This gives a big advantage to children living in an endemic malaria area. All cases of HbS are caused by an identical genetic change occurring under high pressure of malaria infection. It appears to have arisen spontaneously at least five times over our evolution in different regions in Africa, India and the Middle East.

Explanation:

Malaria has caused humans to modify the red cell to protect itself from infection. Almost every part of the red cell, from its membrane to the globin genes that confer its role in oxygen transport harbor common genetic changes in a desperate effort to help our species survive the onslaught of malaria.