Describe the relationship between genes, alleles and phenotype in single gene inheritance.

Eduqas GCSE Biology — 2.4.1 Variation · Describe · 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 section of DNA that codes for a particular protein or characteristic.
Alleles are different versions of the same gene, differing in their DNA base sequences.
A dominant allele is expressed in the phenotype whenever it is present; a recessive allele is expressed only when two copies are present or when no dominant allele is present.
The phenotype is the observable physical characteristic produced by the interaction between the genotype (the alleles present) and the environment.

Examiner tips

  • Use the exact terms ‘gene’, ‘allele’, ‘dominant’, ‘recessive’, ‘phenotype’, ‘genotype’. Show the logical order: gene → alleles → dominance → phenotype. Keep the answer concise, 4–5 short sentences.
  • common_mistakes
  • :
  • Confusing genotype with phenotype. Omitting the role of the environment in determining phenotype. Using vague terms like ‘trait’ instead of ‘phenotype’.

Mark scheme (4 marks)

  1. A gene is a section/length of DNA that codes for a particular protein / characteristic.
  2. Alleles are different versions of the same gene (which may have slightly different DNA base sequences).
  3. A dominant allele is always expressed in the phenotype when present; a recessive allele is only expressed when two copies are present / in the absence of the dominant allele.
  4. The phenotype is the physical/observable characteristic of an organism produced by the interaction between the genotype (alleles present) and the environment.

Key terms in this question

gene · allele · phenotype

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