In the context of human genetic variation, sickle cell anemia is primarily caused by which type of mutation in the HBB gene?

• A. Nonsense mutation (single-nucleotide substitution that creates a premature stop codon)
• B. Frameshift mutation (insertion or deletion that alters the reading frame)
• C. Chromosomal inversion (reversal of a large segment within a chromosome)
• D. Missense point mutation (single-nucleotide substitution that changes one amino acid)

Answer: (D)