Step 1: Understanding the Problem:
Mutations are alterations in the DNA sequence. A point mutation is a highly specific type of mutation where only a single nucleotide base is changed, inserted, or deleted from the genetic sequence.
Step 2: Approach and Formula:
Recall classical genetic disorders and identify which one is primarily characterized and widely cited in literature as being caused by a single base-pair substitution.
Step 3: Detailed Explanation:
Sickle cell anaemia is universally cited as the textbook classical example of a point mutation.
The disease originates from a specific genetic defect: a single nitrogenous base substitution in the DNA sequence that codes for the beta-globin chain of the hemoglobin molecule.
Specifically, an Adenine is replaced by a Thymine (CTC $\rightarrow$ CAC on the DNA template strand), causing the resulting mRNA codon to change from GAG to GUG.
This seemingly minor single-base change substitutes the amino acid Glutamic acid with Valine at the 6th position of the beta-globin chain, profoundly altering the shape and function of red blood cells under low oxygen conditions.
Step 4: Final Answer:
Sickle cell anaemia is the classical example.