Sickle cell disease is caused by what type of genetic change?

• A. A point mutation on chromosome 11
• B. A chromosomal deletion on chromosome 11
• C. A frameshift mutation on chromosome 1
• D. A duplication on chromosome 20

Answer: (A)

A single-nucleotide substitution in the beta-globin gene (HBB) on chromosome 11 causes sickle cell disease. This is a missense point mutation, where one base change alters a codon so that the sixth amino acid of the beta-globin chain changes from glutamic acid to valine. That small change produces hemoglobin S, which tends to polymerize under low oxygen and makes red blood cells become stiff and sickle-shaped. This differs from a chromosomal deletion (removing genetic material), a frameshift mutation (insertions or deletions that shift the reading frame and alter many downstream amino acids), or a duplication (adding extra copies of genetic material). So the described genetic change is a point mutation on chromosome 11.