thalassemia
Hemoglobin, the oxygen-carrying protein in red blood cells, is built from two kinds of matching globin chains, alpha and beta, that must be made in balanced amounts. In thalassemia the factory under-produces one of those chains, so the chains are mismatched and the red cells are small, pale and short-lived. The result is anemia that ranges from mild to severe depending on how much production is lost.
Thalassemias are inherited, usually recessive disorders grouped by which chain is affected: alpha-thalassemia involves the alpha-globin genes, and beta-thalassemia the beta-globin (HBB) gene. Unlike sickle-cell disease, which changes the shape of the hemoglobin molecule, thalassemia is mainly a problem of quantity, too little of a normal chain, often caused by deletions or by mutations that reduce gene expression.
Thalassemias are among the most common single-gene disorders in the world, concentrated in regions where malaria has been common, because carriers gain some protection, another example of balancing selection. Severe forms may need regular transfusions; carrier testing and prenatal diagnosis are widely used in high-prevalence areas. This entry is educational, not medical advice.
A routine blood count shows small, pale red cells with mild anemia in a person from a Mediterranean background; further testing reveals beta-thalassemia trait, a carrier state that is usually harmless on its own.
Carrier (trait) states are common and usually mild, but matter for family planning.
Sickle-cell disease and thalassemia both affect globin but differently: one alters the protein's quality (a changed amino acid), the other its quantity (too little chain). Comparing them clarifies how mutations can harm in different ways.