© 2007 Rector and Visitors of the University of VirginiaCharles E. Hess, M.D. and Lindsey Krstic, B.A.
Sickle cell disease is present in about 80,000 Americans. It is the most common inherited blood disorder in the United States, though many are only carriers. Being a carrier, however, doesn’t mean there will be the presence of the sickle cell trait.
The sickle cell trait means, in simple terms, the production of abnormal hemoglobin on red blood cells. Hemoglobin distributes oxygen from the lungs to the rest of the body; a means of existence. The red blood cells, if affected by the sickle cell disease become constricted and sickle-like, making it difficult to travel through blood vessels, especially because many can clump together forming clots.
The physical expression of the disease only happens when a person is homozygous for the trait, meaning they have the same two alleles present for the trait. Those who are carriers, are heterozygous for the trait, meaning they only have one allele for the disease that is paired with a normal allele.
In malaria conceiving parts of the world, however, the presence of a sickle cell allele can mean saving their life if they contract malaria.
Malaria lives off of the hemoglobin on red blood cells. A person that has the sickle cell trait (that is, the homozygous form of the trait) will most definitely not be able to get malaria because of the abnormal hemoglobin. However, their chances of survival decreases anyway because the outcomes of expressing the sickle cell trait are lung tissue damage causing acute chest syndrome, stroke, damages to the spleen, kidney and liver, all of which can lower immune system health making the person vulnerable to bacterial infections. A person with the expressed sickle cell trait is therefore selected against in natural selection because of the detrimental affects of the disease.
Because a person is born with it, if homozygous for the disease, many die during childhood. Due to technology, research and preventative drug treatments, some sickle patients have been known to live past the age of 50.
In malarial areas, if a person does not have any alleles for sickle cell disease, then they have no natural way to fight it off, and are therefore selected against via malaria.
In the best of both worlds, however, there is the balanced polymorphism of the trait. A balanced polymorphism is the maintenance of a trait due to the selective advantage of a heterozygote.
The heterozygote for sickle cell disease (again a person who is a carrier) displays a balanced polymorphism. This is because the person does express the trait for sickle cell disease, and are therefore not affected by the trait.
The coolest part of this balanced polymorphism, though, is its ability to still protect people from malaria. If malaria is found in the body, the one allele present for sickle cell disease switches producing the abnormal hemoglobin. Well, if the hemoglobin on red blood cells isn’t available for malaria to support itself and spread throughout the body, then it will die out and rid itself of the body. As soon as it is gone, the allele for the sickle cell trait turns off, and the production of hemoglobin returns to normal.
The carrier is therefore well protected if living in a malarial environment. However, they can pass it on to their offspring, and if two people carrying an allele for sickle cell disease mate, there is a 25 percent chance they will have a child born expressing the sickle cell trait.
African Americans and Hispanic Americans are the most frequent carriers in the United States at one and 12 and one in 100, respectively.
The heterozygous form of sickle cell disease is just one example of a balanced polymorphism displaying the many marvels of human evolution.
