**Core Concept:** Hemoglobin (Hb) is a protein found in red blood cells (RBCs) that carries oxygen from the lungs to the body's tissues and returns carbon dioxide back to the lungs. It consists of two alpha chains, two beta chains, and two gamma chains. Hemoglobinopathies are disorders affecting the structure of these chains, leading to abnormal hemoglobin molecules. Sickle cell anemia is a common example caused by a mutation in the beta-globin chain.

**Why the Correct Answer is Right:** In Hemoglobin S (HbS), the mutation involves the beta-globin chain, specifically the Glutamic acid residue at position 6 is replaced by Valine (also known as beta-thalassemia trait). This alteration results in an abnormal hemoglobin molecule that can polymerize under low oxygen conditions, causing RBCs to become rigid and biconcave in shape (sickling). This leads to reduced oxygen-carrying capacity, vaso-occlusion, and tissue hypoxia.

**Why Each Wrong Option is Incorrect:**

A. False: The question asks about electrophoretic mobility of HbS, not alpha chains.
B. False: The mutation in HbS affects the beta-globin chain, not alpha chains.
C. False: HbS has a different electrophoretic mobility than the other hemoglobin variants, but this question asks about HbS specifically.
D. False: The question asks about electrophoretic mobility, not the clinical consequences of the mutation.

**Clinical Pearl:** Electrophoresis is a crucial diagnostic tool for identifying abnormal hemoglobin variants like HbS. By analyzing the electrophoretic mobility of hemoglobin fractions, medical professionals can diagnose and manage patients with hemoglobinopathies, ensuring appropriate treatment and counseling for genetic carriers.