The beta-globin gene mutation in sickle cell disease is best described as which type of mutation?

• A. Deletion of beta-globin gene
• B. Point mutation causing substitution of valine for glutamic acid at position 6 of beta-globin
• C. Insertion of valine at position 6
• D. Nonsense mutation in beta-globin

Answer: (B)

A single nucleotide change in the beta-globin gene alters one amino acid in the protein, specifically substituting valine for glutamic acid at position 6. This is a missense point mutation, where a base pair change changes the codon so that the amino acid encoded is different. The result is abnormal hemoglobin (hemoglobin S) that tends to polymerize under low oxygen, deforming red blood cells and causing sickle cell disease. Other mutation types described—deletion, insertion, or a nonsense (premature stop)—would have different, broader effects, such as removing part of the protein, adding extra amino acids, or truncating the protein, none of which matches the precise Glu6Val change seen in this condition.