Final answer:
The abnormal hemoglobin seen in sickle cell disease is due to a point mutation in the beta chain of the hemoglobin molecule, where valine replaces glutamic acid, causing hemoglobin and red blood cells to distort into a sickle shape.
Step-by-step explanation:
The abnormal hemoglobin associated with sickle cell disease results from a point mutation in the beta (b) chain of the hemoglobin molecule. This mutation is a single base change that leads to a single amino acid substitution, where valine replaces glutamic acid at the 6th position in the beta chain of hemoglobin, specifically. This alteration results in hemoglobin molecules that can link together under low-oxygen conditions, forming rigid structures that change the red blood cells into a sickle shape. The sickled cells cause various health complications because they are less flexible and can block blood flow, leading to oxygen deprivation in tissues.