HbH disease in alpha-thalassemia occurs with how many defective alpha-globin genes?

HbH disease in alpha-thalassemia occurs with how many defective alpha-globin genes?

• A. One defective gene
• B. Three defective genes
• C. Two defective genes
• D. Four defective genes

Answer: (B)

HbH disease arises when three of the four alpha-globin genes are defective. Humans have two alpha-globin genes on each chromosome 16, totaling four. With three genes nonfunctional, there isn’t enough alpha-globin to pair with the beta-like chains, so excess beta chains form tetramers (HbH, beta4). These HbH tetramers cause hemolysis and anemia characteristic of the condition. If only one gene is defective, it's usually a silent carrier; two defective genes cause a mild alpha-thalassemia trait; all four defective lead to Hb Barts hydrops fetalis.

HbH disease arises when three of the four alpha-globin genes are defective. Humans have two alpha-globin genes on each chromosome 16, totaling four. With three genes nonfunctional, there isn’t enough alpha-globin to pair with the beta-like chains, so excess beta chains form tetramers (HbH, beta4). These HbH tetramers cause hemolysis and anemia characteristic of the condition. If only one gene is defective, it's usually a silent carrier; two defective genes cause a mild alpha-thalassemia trait; all four defective lead to Hb Barts hydrops fetalis.