Explain why two organisms of the same species can have different characteristics even though they share the same genome.

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).

Scientists have found that identical twins, who have the same DNA sequence, can sometimes develop different physical characteristics as they grow older.

Model answer (4 marks)

Gene expression can differ between individuals, so not all genes are expressed in every cell or at every time.
Environmental factors influence which genes are expressed.
Gene expression leads to the production of proteins, and proteins determine physical characteristics.
If different genes are expressed, different proteins are made, giving different phenotypes despite the same genotype.

Examiner tips

Use the word ‘gene expression’ and ‘proteins’ – these are key terms. Show the causal chain: gene expression → proteins → characteristics. Mention the role of the environment to hit the environmental factor point. Keep the answer concise – 4 short sentences will cover all marks.

Common mistakes

Confusing DNA sequence with DNA expression; students say ‘different DNA’ instead of ‘different expression’. Omitting the environmental influence or the protein step. Writing long, unfocused paragraphs that miss one of the four points.

Mark scheme (4 marks)

Gene expression can differ between individuals / not all genes are expressed in every cell or at every time
The environment can influence which genes are expressed / environmental factors affect gene expression
Proteins are produced as a result of gene expression, and it is proteins that determine characteristics
If different genes are expressed, different proteins are made, leading to different characteristics / phenotype differs despite same genotype

Key terms in this question

genome · characteristics