What is a defect that causes hemoglobin to become sharp?

A defect that causes hemoglobin to become sharp is known as sickle cell disease, which results from a mutation in the HBB gene that encodes the beta-globin subunit of hemoglobin. This mutation leads to the production of hemoglobin S (HbS), which polymerizes under low oxygen conditions, causing red blood cells to assume a rigid, crescent or "sickle" shape. These sickled cells can obstruct blood flow and lead to various complications, including pain crises and organ damage.