Explain how a mutation in a gene may lead to a change in the phenotype of an organism.

Eduqas GCSE Biology — 2.4.1 Variation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A mutation is a random change in the DNA base sequence of a gene.

The altered base sequence changes the codons that are read during translation, so the amino‑acid sequence of the protein is altered.

The new amino‑acid sequence can give a protein with a different structure or make it non‑functional.

A change in the protein’s structure or function changes the organism’s characteristic, i.e. its phenotype.

Examiner tips

Use the exact terms: mutation, DNA base sequence, codon, amino‑acid, protein, phenotype.
Show the causal chain: mutation → altered codon → altered protein → altered phenotype.
Keep each point short and to the point to fit the 4 marks.

Common mistakes

Confusing mutation with natural variation or selection.
Using vague words like ‘change’ without specifying DNA, codon, protein or phenotype.
Omitting the step that the altered amino‑acid sequence can make the protein non‑functional.

Mark scheme (4 marks)

A mutation is a random change in the DNA (base sequence) of a gene.
A change in the base sequence alters the sequence of amino acids coded for (via the triplet code).
A different amino acid sequence produces a different (or non-functional) protein.
A change in the protein produced leads to a change in the characteristic / phenotype of the organism.

Key terms in this question

mutation · gene · phenotype