Which statement best describes the composition of Hb Barts?

• A. Four gamma chains
• B. Two gamma and two delta chains
• C. Four delta chains
• D. One gamma and three beta chains

Answer: (A)

Hb Barts is a tetramer made up of four gamma globin chains. In the fetus, normal HbF is alpha2 gamma2, but when alpha-globin production is absent (as in severe alpha-thalassemia), gamma chains don’t have alpha partners and instead pair with each other to form gamma4, the Hb Barts tetramer. This structure has very high oxygen affinity and poor release to tissues, so it can’t effectively deliver oxygen. That’s why the statement describing four gamma chains exactly captures Hb Barts. Other combinations describe different hemoglobins (for example HbF is two gamma and two alpha; HbA2 is two alpha and two delta; beta tetramers appear in other conditions) and do not represent Hb Barts.