In which disease, the substitution of amino acid in the globin protein result from a single base substitution at the sixth codon?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Sickle-cell anaemia
(1) Phenylketonuria — Incorrect. In this disease the enzyme that changes phenylalanine into tyrosine is missing. Phenylalanine builds up in the body and harms the brain. Globin is not involved.
(2) Thalassemia — Incorrect. Here the rate of making one of the globin chains (alpha or beta) is reduced. No amino acid is replaced.
(3) Sickle-cell anaemia — Correct. The base change from GAG to GUG at the sixth codon of the beta globin gene puts valine in place of glutamic acid at the sixth position of the beta globin chain. This single change makes the haemoglobin stick together in low oxygen, and the red blood cells become sickle shaped.
(4) Color Blindness — Incorrect. This is a sex-linked recessive defect in the red or green cone cells of the eye. It is not a change in globin.
NCERT Extract:
Mendelian Disorders – …
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