Explain how a mutation in a gene can lead to a change in an organism's phenotype.

Edexcel GCSE Biology (1BI0) — 3.5 Variation and mutation · Explain · 4 marks · View as Markdown

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

Mutations occur randomly in organisms and can have a range of effects. Some mutations alter an organism's observable characteristics whilst others have no noticeable effect.

Model answer (4 marks)

A mutation is a random change in the base sequence of DNA.
A gene codes for the production of a specific protein; the altered base sequence may change the protein produced.
The altered protein may not function correctly or may have a different shape or function.
This change in protein leads to a change in the organism's observable characteristics, i.e. its phenotype.

Examiner tips

Use the exact wording from the mark scheme – e.g. ‘random change in the base sequence of DNA’. Show the causal chain: mutation → altered DNA → altered protein → altered phenotype. Keep each point short and to the point to fit the 4‑mark limit.

Common mistakes

Mixing up ‘gene’ and ‘protein’ – students sometimes say the mutation changes the protein directly without mentioning the gene. Using vague terms like ‘different’ instead of ‘different shape or function’. Failing to mention that the mutation is random or that it affects the DNA sequence.

Mark scheme (4 marks)

A mutation is a random change in the base sequence of DNA
A gene codes for the production of a specific protein / the altered base sequence may change the protein produced
The altered protein may not function correctly / may have a different shape or function
This change in protein leads to a change in the organism's observable characteristics / phenotype

Key terms in this question

mutation · gene · phenotype