Question

Sickle cell anemia is:-

• A. An autosomal linked dominant trait
• B. Caused by substitution of valine by glutamic acid in the $\beta$-globin chain of haemoglobin
• C. Caused by a change in a single base pair of DNA
• D. Characterized by elongated sickle like RBCs with a nucleus

Answer 1

The correct option is C Caused by a change in a single base pair of DNA

Hemoglobin has four protein subunits, two $\alpha$-globin and two $\beta$-globin. The Hb${}^b$ gene codes the $\beta$-chain. Missense mutation, replacement of A by T at the 17th nucleotide of the Hb${}^b$ gene changes the codon GAG (glutamic acid) to GTG (which encodes valine). The mutated allele Hb${}^s$ encodes the abnormal haemoglobin molecules which stick to one another and cause stiffness and sickle shape of red blood cells. The haemoglobin-$\beta$ gene is located on chromosome 11 (autosomal) and the heterozygotes have normal RBCs which means that disease is governed by the recessive gene.

Thus, the correct answer is option C.