On which chromosome is the mutation associated with sickle cell disease located?

• A. Chromosome 7
• B. Chromosome 4
• C. Chromosome 19
• D. Chromosome 11

Answer: (D)

Understanding where a disease-causing gene sits on a chromosome is key. The sickle cell mutation is in the beta-globin gene (HBB), which is located on chromosome 11. A single base change in this gene alters one amino acid in the beta-globin protein of hemoglobin, swapping glutamic acid for valine. This small change makes deoxygenated hemoglobin polymerize, causing red blood cells to sickle and leading to the disease’s symptoms. Since the HBB gene resides on chromosome 11, chromosome 11 is the correct location. The other chromosomes listed do not contain this gene, so they don’t explain this mutation.