Which chromosome carries the sickle cell mutation?

• A. Chromosome 11
• B. Chromosome 7
• C. Chromosome 21
• D. Chromosome X

Answer: (A)

The key idea is that a gene’s location on a chromosome determines where a mutation is found. The sickle cell mutation sits in the beta-globin gene (HBB) within the beta-globin gene cluster on chromosome 11. A single nucleotide change in this gene alters the amino acid in the hemoglobin beta chain from glutamic acid to valine, producing hemoglobin S that tends to polymerize and cause red blood cells to sickle under low oxygen. Since this gene is on chromosome 11, chromosome 11 is the one carrying the mutation. The other chromosomes listed do not contain the HBB gene, so they aren’t the source of this mutation.