Sickle-cell anaemia is a Mendelian disorder which can be transmitted from parents to the offspring. Which of the following statements are applicable to this trait?
(A) Autosome linked dominant trait
(B) The defect is caused by the substitution of glutamic acid by valine.
(C) Caused due to single base substitution of beta globin gene from GUG to GAG.
(D) The mutant haemoglobin undergoes polymerisation under low oxygen tension and changes the shape of RBC from biconcave to sickle like structure.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
Sickle-cell anaemia is caused by a change in the gene for the β chain of haemoglobin.
(A) Autosome linked dominant trait — Incorrect. It is an autosome-linked recessive trait. Only a person with two mutant alleles (HbSHbS) is diseased. A heterozygous person is a carrier.
(B) The defect is caused by the substitution of glutamic acid by valine. — Correct. This change happens at the sixth position of the β globin chain.
(C) Caused due to single base substitution of beta globin gene from GUG to GAG. — Incorrect. The change runs the other way. The sixth codon changes from GAG (glutamic acid) to GUG (valine).
(D) The mutant haemoglobin undergoes polymerisation under low oxygen tension and changes the shape of RBC from biconcave to sickle like structure. — Correct. Under low oxygen the mutant haemoglobin forms long chains, and the red cell turns from a biconcave disc into a sickle shape.
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, HbA and HbS. Out of the three possible genotypes only homozygous individuals for HbS (HbSHbS) show the diseased phenotype. Heterozygous (HbAHbS) 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. 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. The mutant haemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of the RBC from biconcave disc to elongated sickle like structure (Figure 4.15).”
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