Sickle-cell anemia is caused due to:

The correct answer is Option (1) → Change of amino acid in β-chain of haemoglobin

Haemoglobin has two kinds of globin chains, α and β. In sickle-cell anaemia, a single base change in the gene for the β-globin chain puts valine in place of glutamic acid at the sixth position. The red cells then turn sickle-shaped at low oxygen.

Option (1) Change of amino acid in β-chain of haemoglobin — Correct. Only the β-globin chain is changed, by one amino acid. A single base change (GAG to GUG) in its gene causes it.

Option (2) Change of amino acid in α-chain of haemoglobin — Incorrect. The α-chain is normal in sickle-cell anaemia. The mutation lies in the gene for the β-globin chain, so only that chain is changed.

Option (3) Change of amino acid in both α- and β-chains of haemoglobin — Incorrect. One gene, one chain: the α-chain is not affected.

Option (4) Change of amino acid in one of the α- or β-chains of haemoglobin — Incorrect. It leaves the chain open; the change is always in the β-chain.

NCERT Extract:
Mendelian Disorders – “… Sickle-cell anaemia : This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous). The disease is controlled by a single pair of allele, Hb^A and Hb^S. Out of the three possible genotypes only homozygous individuals for Hb^S (Hb^SHb^S) show the diseased phenotype. Heterozygous (Hb^AHb^S) individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure 4.15). 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.”

Figure 4.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion of β-chain of haemoglobin: (a) From a normal individual; (b) From an individual with sickle-cell anaemia