Any of a group of disorders caused by genetic abnormality of the hemoglobin molecule. The most prominent types are sickle-cell anemia and thalassemia, a set of disorders whose symptoms range from none to fatal anemia.
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In general a sickling test (sodium bisulfite) is performed on abnormal hemoglobins migrating in the S location to see if the red cells precipitate in solution.
This is how abnormal Hgb variants are isolated and identified using these two methods. For example a Hgb G-Philadelphia would migrate with S on alkaline electrophoresis and would migrate with A on acid electrophoresis, respectively.
Some hemoglobinopathies (and also related diseases like glucose-6-phosphate dehydrogenase deficiency) seem to have given an evolutionary benefit, especially to heterozygotes, in areas where malaria is endemic. Malaria parasites live inside red blood cells, but subtly disturb normal cellular function. In patients predisposed for rapid clearance of red blood cells, this may lead to early destruction of cells infected with the parasite and increased chance of survival for the carrier of the trait.