HbA2 has
**Core Concept**
HbA2 is a minor component of hemoglobin, making up approximately 2-3.5% of the total hemoglobin in adults. It is a variant of hemoglobin A (HbA) and is composed of two alpha and two delta chains. Understanding the composition and structure of HbA2 is crucial for diagnosing and managing disorders related to hemoglobin.
**Why the Correct Answer is Right**
HbA2 is a beta-zero (β0) thalassemia minor, which is characterized by reduced production of the beta-globin chains. This results in an increase in the production of delta-globin chains, leading to the formation of HbA2. The delta chains are more stable than the beta chains, which is why HbA2 is more resistant to degradation. The increased production of HbA2 in beta-zero thalassemia minor leads to its elevated levels in the blood.
**Why Each Wrong Option is Incorrect**
**Option A:** HbA2 is a major component of hemoglobin. Incorrect because HbA2 makes up only a small percentage of the total hemoglobin in adults.
**Option B:** HbA2 is composed of two alpha and two beta chains. Incorrect because HbA2 is composed of two alpha and two delta chains, not beta chains.
**Option C:** HbA2 is associated with alpha-thalassemia. Incorrect because HbA2 is associated with beta-zero thalassemia minor.
**Clinical Pearl / High-Yield Fact**
HbA2 levels are often used as a diagnostic tool for beta-zero thalassemia minor, as they are typically elevated in this condition. A normal HbA2 level is less than 3.5%, while an elevated level is often greater than 3.5%.
**Correct Answer: D. HbA2 is a beta-zero (β0) thalassemia minor.**