IB Biology SL 4.1.5 Evidence for Natural Selection

The sickle-cell hemoglobin allele $\left(\mathrm{Hb}^{\mathrm{s}}\right)$ has a high frequency in regions where there is malaria. This is believed to be due to a selective advantage against fatal malaria for the heterozygote ( $\mathrm{Hb}^{\mathrm{A}} \mathrm{Hb}^{\mathrm{s}}$ ). Individuals who are homozygous for the sickle cell allele ( $\mathrm{Hb}^{\mathrm{s}} \mathrm{Hb}^{\mathrm{s}}$ ) have sickle cell anemia while heterozygotes have a much less severe form of the disease. Normal individuals without sickle cell anemia are designated $\mathrm{Hb}^{\mathrm{A}} \mathrm{Hb}^{\mathrm{A}}$.

This study involved 1022 children living in an area of Kenya where malaria is present. The possible protective effect of the sickle cell allele against mortality due to malaria was investigated. The data show the percentage of children surviving after birth over a period of 48 months.