Statement I: Sickle-cell anaemia is caused by the substitution of glutamic acid by valine at the sixth position of the β-globin chain.
Statement II: β-thalassemia is caused by a mutation in the HBB gene located on chromosome 16.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Only Statement I is correct.
Statement I: Sickle-cell anaemia is caused by the substitution of glutamic acid by valine at the sixth position of the β-globin chain. — True. A single base change in the sixth codon of the beta globin gene (GAG to GUG) puts valine in place of glutamic acid. The faulty haemoglobin polymerises at low oxygen tension and the RBCs become sickle-shaped.
Statement II: β-thalassemia is caused by a mutation in the HBB gene located on chromosome 16. — False. β-thalassemia is controlled by the single gene HBB, but this gene lies on chromosome 11. Chromosome 16 carries HBA1 and HBA2, the two genes of α-thalassemia.
NCERT Extract:
Mendelian Disorders – “… The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.”
Mendelian Disorders – “… α Thalassemia is controlled by two closely linked genes HBA1 and HBA2 on chromosome 16 of each parent and it is observed due to mutation or deletion of one or more of the four genes. The more genes affected, the less alpha globin molecules produced. While β Thalassemia is controlled by a single gene HBB on chromosome 11 of each parent and occurs due to mutation of one or both the genes.”