Which of the following disorder is an example of point mutation?

The correct option is Sickle cell anaemia.

To understand why this is the correct answer, let's break down the genetic basis of the disorder step-by-step:

1. What is a Point Mutation?
A point mutation is a type of genetic mutation where a single nucleotide base in the DNA sequence is altered, inserted, or deleted. This single-base change can lead to the substitution of one amino acid for another in the resulting protein, potentially altering its structure and function.

2. The Genetic Basis of Sickle Cell Anaemia
Sickle cell anaemia is a classic textbook example of a point mutation in humans. It affects the gene that codes for the 𝛽-globin chain of hemoglobin (the oxygen-carrying protein in red blood cells). The specific mutation occurs as follows:
At the molecular level, the codon for the sixth amino acid of the 𝛽-globin chain is mutated.
The normal DNA codon is GAG, which codes for the amino acid glutamic acid.
A point mutation occurs where the middle nucleotide thymine (T) is replaced by adenine (A) in the template strand (transcribing to a change in mRNA from GAG to GUG).
The mutated codon GUG codes for the amino acid valine instead of glutamic acid.

This single amino acid substitution (glutamic acid to valine at the 6th position) causes the hemoglobin molecules to polymerize under low oxygen conditions, distorting the red blood cells into a sickle shape.

3. Analyzing the Other Options:
- Down’s syndrome: This is a chromosomal disorder (specifically trisomy 21) caused by the presence of an extra copy of chromosome 21, not a point mutation.
- Night blindness: This is typically a nutritional deficiency disorder (caused by lack of Vitamin A) or can be associated with various complex genetic conditions, but it is not classified fundamentally as a single-gene point mutation disease like sickle cell anaemia.
- Thalassemia: While also a disorder affecting hemoglobin synthesis, thalassemia is generally caused by the deletion of one or more genes or larger genetic rearrangements leading to reduced synthesis of globin chains, rather than a classic single-base point mutation producing an abnormal variant of hemoglobin.