Explain why sickle-cell trait is advantageous in some regions of the world.
Sickle-cell trait refers to the heterozygous condition where a person has one defective gene for sickle cell and one gene for a normal red blood cell. Due to incomplete dominance, heterozygous (carriers of the sickling gene) show some signs of sickling and produce some sickle cells, which does not normally cause problems for the individual under normal conditions. Problems arise for heterozygotes if they are in low oxygen conditions (high altitudes) or if they are dehydrated. Heterozygotes have increased resistance to malaria. Malaria is caused by a protozoan that invades red blood cells. In malarial areas of the world, heterozygots for sickle-cell anaemia have increases resistance to malaria. Thus the sickle-cell gene is present in higher frequencies in malarial areas that would be expected for a recessive gene that reduces life expectancy.