Sickle cell anaemia (SCA) is a genetic disease which causes the red-blood cells of a sufferer to be shaped like sickles, insteaed of the normal rounded shape. This causes the cells to become stuck in capillaries and deprives the body of oxygen, eventually leading to damage of some organs, stroke, or anemia.
The gene allele responsible for SCA is incompletely recessive. It is a known mutation of a single nucleotide of the beta globin gene, preventing the hemoglobin protein from folding properly. If two parents who are carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of their child being a carrier.
The sufferers of the illness usually die early. Still, the disease has not died out. This is because carriers of the gene for the illness are resistant to malaria. Carriers of the disease have an unsymptomatic condition called Sickle Cell Trait. Since the gene is incompletely recessive, carriers have a few sickle red blood cells at all times but not enough to cause symptoms. only if they are deproed of Oxygen (for example, climbing a mountain) will they develop the disease. The presence of the gene makes the blood cells fragile. The malaria parasite has a complex life cycle and spends part of it in red blood cells. In a carrier, the presence of the malaria parasite causes the red blood cell to rupture, making the malaria unable to reproduce. In areas where this is a problem, such as Africa, people's chances of survival actually increase if they carry SCA. So the illness continues and is especially prevalent among people with recent ancestry in malaria striken areas, such as Africa, the Mediterranean, India and the Middle East. In fact, SCA is the most common genetic disorder among African-Americans; about 1 in every 13 African-Americans are carriers.
People who are known carriers of the disease often undergo genetic counselling before they have a child. A test to see if the unborn child has the disease takes either a blood sample or a sample of amniotic fluid. Since taking a blood sample from a fetus is dangerous, the latter test is usually used.
After the gene for this disease was discovered in 1979, the US Air Force recquired African American applicants to test for the gene. It dismissed 143 applicants because they had the gene, though none of them had the condition. It eventually withdrew the recquirement, but only after a trainee filed a lawsuit. Now, some insurance companies are doing the same thing to eliminate, in thier terminology, 'unwise investments.'
See also: Dominance Relationships Genetic Counselling