Explain why not all mutations 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 are random changes in the base sequence of DNA. Scientists studying a population of wild mice found that, despite many mutations being present across individuals, very few of these mutations produced any observable difference in the mice.

Model answer (4 marks)

A mutation may be neutral and have no effect on the organism.
A mutation in a non‑coding region of DNA does not code for a protein and therefore does not affect protein production.
Even if a gene is mutated, the same amino acid may still be produced – the mutation may be silent.
If the protein or enzyme produced is unchanged, the phenotype will not change.

Examiner tips

  • Use the exact terms: ‘neutral’, ‘non‑coding region’, ‘silent mutation’, ‘unchanged protein’.
  • Show the logical chain: mutation → no effect on protein → no phenotype change.
  • Keep each point short and to the point.

Mark scheme (4 marks)

  1. A mutation may be neutral / have no effect on the organism
  2. A mutation in a non-coding region of DNA does not code for a protein / does not affect protein production
  3. Even if a gene is mutated, the same amino acid may still be produced / the mutation may be silent
  4. If the protein / enzyme produced is unchanged, the phenotype will not change

Key terms in this question

mutation · phenotype

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