It is important to note that even genetic diseases are caused by multiple social, environmental and biological factors. This is particularly true in the case of sickle-cell anemia, a genetically inherited blood disorder usually found in people with West African ancestry (PBS 2001). The following paper will summarize the genetic, geographic and cultural factors that contribute to the incidence of sickle-cell anemia and why this disease is called a “balanced polymorphism”.
Sickle cell anemia is a fatal genetic disease that is caused by a recessive Mendelian genetic defect. Those with the homozygous sickle-cell gene mutation in the β-globin (“beta globin”; also known as “hemoglobin, beta” or “HBB” ) genetically code for an abnormal version of HBB called “hemoglobin S” or “Hbs” and suffer from sickle-cell anemia (Lister Hill National Center for Biomedical Communications 2012). Specifically the mutation changes a single amino acid in the DNA from glutamine to valine at location 6 in beta globin or “Glu6Val” (Lister Hill National Center for Biomedical Communications 2012). Normal human hemoglobin is made up of two beta chains (formerly known as “hemoglobin, beta” or by the symbol HBB) and two alpha chains (formerly known as “hemoglobin, alpha” or by the symbol HBA) of globin. The heterozygous form of the disease, sickle-cell trait, confers immunity to the deadly malaria virus. Sickle cell trait is generally not deadly but can become life threatening to its carriers who experience insufficient oxygen from high altitude or extreme physical exertion (O’Neill 2011).
The highest frequency of sickle-cell disease world-wide is found in malarial regions of equatorial Africa where an average of 35% of the population are carriers of the sickle-cell genetic mutation (Faeman 2000:162). In North America, about 1 in 12 African Americans are carriers of the sickle-cell mutation and about 72,000 of them are homozygous for it (and have sickle-cell anemia disease). Most African Americans in North America have ancestors from malarial zones of West and Central Equatorial Africa (O’Neill 2011).
Sickle cell trait is a human blood disorder that was first selected by biocultural factors in equatorial Africa about 2,000 years ago with the introduction of slash and burn horticulture. Clearing land for root crops in the tropical region altered the environment and provided an attractive habitat for the Anopheles gambiae group of mosquitoes, vectors for the malaria virus. As the human population density increased, the human food supply for the mosquitoes increased and malaria spread rapidly throughout equatorial Africa. Since the heterozygous form of the sickle-cell mutation provided valuable immunity from the deadly malaria virus, the sickle-cell allele was selected for the past 2,000 years.
Sickle cell anemia is called a “balanced polymorphism” because it’s caused by a genetic mutation that is kept at a generally constant ratio in a population due to selection pressures. It is polymorphic because it’s caused by the mutated form of the β-globin (“beta globin”) gene that is found at more than one loci on the chromosome that codes for blood cell proteins (Jurmain 2010:431). The “balanced” label is applied because individuals who are heterozygous for the sickle-cell gene mutation have a resiliency to the deadly malaria virus so it is selected for in regions with malaria. As a consequence, they are able to survive malaria and live to reproduce and pass on the sickle-cell genetic mutation on to the next generation. However, the sickle-cell gene mutation will not overtake normal forms of the gene since individuals who are homozygous for the sickle-cell gene mutation generally die from sickle-cell anemia disease before they reach adulthood. Sickle cell anemia is a good example of the cultural and environmental factors of natural selection working on a human population–an example of microevolution. In this case, it was agricultural practices and an increase in human population density that selected for the “sickle-cell trait”– having one copy of the sickle-cell gene mutation.
Faerman, Marina (2000) American Journal of Physical Anthropology, 111: 153-163.
Jurmain, Robert, Lynn Kilgore, Wenda Trevanthan, Russell L. Ciochon (2010) Essentials of Physical Anthropology. Belmont, CA: Wadsworth CENAGE Learning, Inc. Pp.443.
O’Neill, Dennis (2011) “Natural Selection” Palomar College. Electronic document: http://anthro.palomar.edu/synthetic/synth_4.htm Retrieved on February 10, 2012.
Lister Hill Center for Biomedical Communications (2012) National Institutes of Health. Retrieved from: http://ghr.nlm.nih.gov/gene/HBB Retrieved on February 13, 2012.
Nagel, Ronald (2001) WGBH Educational Foundation. Electronic document: http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html retrieved on February 12, 2012.
Thompson, Robert (2006) National Institute of Health. Electronic document: http://www.ncbi.nlm.nih.gov/books/NBK19938/ retrieved February 11, 2012.