In which condition are both beta-globin chains of hemoglobin defective?

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Sickle cell trait (heterozygous)Severe beta-thalassemia (thalassemia major)Sickle cell anemia (homozygous)Megaloblastic anemia

Explanation

Thalassemia major involves mutations affecting both beta-globin chains, leading to severe anemia. In contrast, sickle cell trait has only one abnormal beta chain, and homozygous sickle cell anemia affects the beta chains differently. Megaloblastic anemia is unrelated to beta-globin abnormalities.