Explain how a mutation in a gene may affect the phenotype of an organism.

Eduqas GCSE Biology — 7.3 Variation and evolution · 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.
This changes the sequence of amino acids coded for, altering which amino acids are joined during protein synthesis.
The altered amino acid sequence produces a different protein structure.
The changed protein can modify the organism’s physical characteristics, affecting the phenotype – the effect may be neutral, beneficial or damaging.

Examiner tips

Use the word ‘mutation’ first, then explain its effect on DNA, amino acids, protein and phenotype.
Show the causal chain: DNA change → amino acid change → protein structure change → phenotype change.
Mention that the effect can be neutral, beneficial or damaging to cover all options.

Common mistakes

Confusing mutation with natural selection or genetic drift.
Failing to link the DNA change to a change in protein structure.
Not stating that the phenotype effect can be neutral, beneficial or harmful.

Mark scheme (4 marks)

A mutation is a random change in DNA / the base sequence of DNA
This alters the sequence of amino acids coded for / changes which amino acids are joined together during protein synthesis
This produces a different / altered protein (structure)
The changed protein may alter the physical characteristics of the organism / affect the phenotype; the effect may be neutral, beneficial or damaging

Key terms in this question

mutation · phenotype