Hb Bart's hydrops fetalis results from deletion of alpha-globin genes.

Hb Bart's hydrops fetalis results from deletion of alpha-globin genes.

• A. Four
• B. One
• C. Two
• D. Three

Answer: (A)

Hb Bart's hydrops fetalis occurs when there are no functional alpha-globin chains to pair with the gamma chains present in the fetus. In humans, there are two alpha-globin genes on each chromosome 16 (alpha1 and alpha2), so there are four copies in total. If all four alpha-globin genes are deleted, no alpha chains are produced, and gamma chains form tetramers (Hb Bart’s) that have very high oxygen affinity but deliver oxygen poorly to tissues. This severe imbalance causes profound fetal anemia and hydrops fetalis, often lethal before or shortly after birth. If three alpha-globin genes are deleted, one functional alpha-globin gene remains, so some alpha chains are produced and the fetus typically develops HbH disease (beta4 tetramers) with their own anemia, not the Hb Bart’s hydrops fetalis scenario. Deletions of two or one gene lead to milder forms (alpha-thalassemia trait or silent carrier).

Hb Bart's hydrops fetalis occurs when there are no functional alpha-globin chains to pair with the gamma chains present in the fetus. In humans, there are two alpha-globin genes on each chromosome 16 (alpha1 and alpha2), so there are four copies in total. If all four alpha-globin genes are deleted, no alpha chains are produced, and gamma chains form tetramers (Hb Bart’s) that have very high oxygen affinity but deliver oxygen poorly to tissues. This severe imbalance causes profound fetal anemia and hydrops fetalis, often lethal before or shortly after birth.

If three alpha-globin genes are deleted, one functional alpha-globin gene remains, so some alpha chains are produced and the fetus typically develops HbH disease (beta4 tetramers) with their own anemia, not the Hb Bart’s hydrops fetalis scenario. Deletions of two or one gene lead to milder forms (alpha-thalassemia trait or silent carrier).