The sickle-cell allele, which is recessive, causes anemia but confers resistance to malaria in individuals who possess it. However, homozygous recessive individuals often die from anemia but not from malaria, and homozygous dominant individuals do not have anemia but could die from malaria. Heterozygous individuals have the highest relative fitness.

This is an example of __________.

A. the heterozygote advantage
B. the homozygous dominant advantage
C. the homozygous recessive advantage
D. genetic drift
E. gene flow

Heterozygote advantage is the condition where the heterozygous individual has higher relative fitness than both homozygous dominant and homozygous recessive individual. This means that the heterozygote individual has higher chances of surviving than both the homozygous counterparts. Sickle cell anemia is a recessive inherited disorder in which oxygen carrying hemoglobin has an abnormal structure. Hence, the resultant RBCs are not spherical but have crescent sickle shape. The oxygen carrying capacity of such RBCs is drastically reduced but they are unaffected by malarial parasite due to their abnormal structure.

A person with both the recessive genes for sickle cell would not be able to survive due to insufficient oxygen transport in body.
A person with both the dominant genes would be free of sickle cell anemia but in case of malaria would not be able to survive as the normal RBCs would be hijacked by the parasite.
A hetrerozygote would survive both in malaria and sickle cell condition since he has enough normal RBCs for oxygen transport but also has sickle cell RBCs which are unaffected by malarial parasite.