Red Blood Cells in Standard versus Sickle-Cell phenotypes

    Hemoglobin is a tetrameric molecule formed from two Alpha and two Beta chain subunits. Hemoglobin with two standard Beta-chain alleles (AA) is a globular protein. When two sickle-cell Beta-chain alleles (SS) are present, reduced oxygen tension in the blood causes the hemoglobin molecule to undergo a conformational shift to a crystalline form. The formation of large crystals distorts the shape of the red blood cells, which causes blockage of small capillaries and results in acute, painful infarctive crises. The sickle cells are also more easily broken, which results in anemia.