What is the name given to the substitution of an amino acid in the hemoglobin chain?

The substitution of an amino acid in the hemoglobin chain is classified as a hemoglobinopathy, which refers to a group of disorders characterized by abnormalities in the structure of hemoglobin. Hemoglobinopathies can occur due to mutations in the genes encoding the globin chains, leading to the replacement of specific amino acids. For example, in sickle cell disease, a single amino acid substitution occurs (valine replaces glutamic acid) in the beta-globin chain of hemoglobin, which results in the abnormal shaped red blood cells associated with the disease.

Thalassemia, while also a disorder of hemoglobin, is characterized by reduced or absent production of one of the globin chains rather than a change in the amino acid sequence itself. Polycythemia refers to an increased concentration of red blood cells and is not directly related to changes in the hemoglobin structure. While sickle cell disease is indeed a specific example of a hemoglobinopathy, the broader term for the substitution of an amino acid in the hemoglobin chain is hemoglobinopathy itself. Therefore, the best answer is hemoglobinopathy, as it encompasses a wider range of disorders attributable to these structural defects in the hemoglobin molecule.