__________ is caused by a genetic mutation that results in the production of abnormal hemoglobin. Under low-oxygen conditions, the abnormal beta chains link together and form stiff rods that alter the shape of the RBCs.
In sickle-cell anemia, the havoc caused by the abnormal hemoglobin, hemoglobin S (HbS), results from a change in just one of the 146 amino acids in a beta chain of the globin molecule. This alteration causes the beta chains to link together under low-oxygen conditions, forming stiff rods so that HbS becomes spiky and sharp. This, in turn, causes the red blood cells to become crescent shaped when they unload oxygen molecules or when the oxygen content of the blood is lower than normal. The stiff, deformed RBCs rupture easily and tend to jam up in small blood vessels. These events interfere with oxygen delivery, leaving the victims gasping for air and in extreme pain. Bone and chest pain are particularly severe, and infection and stroke often follow.