Describe how a mutation in a gene could affect the phenotype of an organism.

WJEC GCSE Biology (Wales) — 2.4.1 Variation · Describe · 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.
A change in the DNA sequence alters the codons that are read, so the amino‑acid sequence of the protein changes.
The altered amino‑acid sequence changes the protein’s structure or shape.
The altered protein can change the organism’s physical characteristics – the phenotype – which may be neutral, beneficial or harmful.

Examiner tips

Use the exact terms: mutation, DNA base sequence, codon, amino‑acid sequence, protein structure, phenotype.
Show the logical chain: mutation → codon change → amino‑acid change → protein shape → phenotype.

Common mistakes

Confusing mutation with a gene; describing a mutation as a change in the protein directly without mentioning codons.
Losing the step that the DNA change alters the amino‑acid sequence before affecting phenotype.

Mark scheme (4 marks)

A mutation is a random change in DNA (base sequence / nucleotide sequence)
A change in the DNA base sequence alters the sequence of amino acids coded for (via the triplet code)
This changes the structure / shape of the protein produced
This may change the physical characteristics (phenotype) of the organism — which may be neutral, beneficial or damaging

Key terms in this question

mutation · phenotype