The Heterozygous Advantage
Being homozygous for either sickle cell disease or thalassemia can result in serious illness, but being heterozygous for either condition may actually be beneficial under certain circumstances. Both diseases occur primarily in people who live, or whose ancestors lived, in parts of the world where malaria occurs.
Malaria is spread by a mosquito, but it is caused by plasmodia, single-celled organisms that, during an infection, reproduce inside red blood cells. Before the development of modern sanitation and medicine, malaria was a common cause of death. But people who had either the sickle cell trait or thalassemia minor—people who were heterozygous for either condition—were much more likely to survive an infection than were people homozygous for HbA.
This "heterozygote advantage" meant that these individuals tended to live longer, have children and pass their genes on to the next generation. While some of their children died from thalassemia or sickle cell disease, about half of them were heterozygous and benefited from the heterozygote advantage. This survival advantage explains the high prevalence of these alleles in these populations.
Today, especially in developed countries, there are effective methods for preventing and treating malaria. Nevertheless, the genes for sickle cell disease and thalassemia still exist and are passed down to children who will never be exposed to malaria. It is likely that these genes will very slowly be lost from the gene pool.
Weatherall, D. J. "ABC of Clinical Haematology: The Hereditary Anaemias." British Medical Journal 314 (1997): 492-496.
"Bioelectronics Laboratory Index." Seikei University. <http://www.ee.seikei.ac.jp/user/seiichi/lecture/Biomedical/02/graphics/hemoglobin.gif>.
Facts about Sickle Cell Disease. National Heart, Lung, and Blood Institute. <http://www.nhlbi.nih.gov/health/public/blood/sickle/sca_fact.txt>.
Joint Center for Sickle Cell and Thalassemic Disorders. <http://sickle.bwh.harvard.edu>.
Medicine EncyclopediaGenetics in Medicine - Part 2Hemoglobinopathies - Hemoglobin Structure And Function, The Genetics Of Hemoglobinopathies, Sickle Cell Disease, Treatment Options And Continuing Research