Explain the difference between a gene and an allele, and explain how a change in the base sequence of a gene can affect the protein it codes for.

Eduqas GCSE Biology — 7.1 The genome and gene expression · 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 gene is a specific sequence of DNA that codes for a particular protein or trait.
An allele is an alternative version of that same gene.
A mutation in the gene’s base sequence alters the codons, changing the amino‑acid sequence of the protein.
The altered amino‑acid sequence changes the protein’s shape or active site, so it may lose its normal function.

Examiner tips

Use the exact terms ‘gene’, ‘allele’, ‘mutation’, ‘amino‑acid sequence’, ‘shape/structure’ and ‘active site’.
Show the logical chain: gene → codon → amino acid → protein structure → function.
Keep the answer concise – 4 marks only.
Use UK spelling (e.g., ‘allele’).

Common mistakes

Confusing a gene with a chromosome or a trait.
Using vague terms like ‘different’ instead of ‘alternative version’ for allele.
Failing to mention the effect on the protein’s shape or active site.

Mark scheme (4 marks)

A gene is a section/sequence of DNA that codes for a specific protein / particular characteristic
An allele is a different version/form of the same gene
A change/mutation in the base sequence of a gene changes the sequence of amino acids in the protein / changes which amino acids are joined together
This changes the shape/structure of the protein so it may not function correctly / the active site of an enzyme changes so it no longer works

Key terms in this question

gene · allele · base sequence · protein