Explain why two parents who both have brown eyes can produce a child with blue eyes. Use the terms dominant, recessive, heterozygous and phenotype in your answer.

Eduqas GCSE Biology — 7.2 Inheritance · Explain · 4 marks · View as Markdown

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

In humans, the allele for brown eyes (B) is dominant over the allele for blue eyes (b).

Model answer (4 marks)

Both parents must be heterozygous (Bb), carrying one dominant and one recessive allele.
The brown eye phenotype is expressed because B (brown) is dominant and masks the recessive b allele.
Each parent can pass either the B or the b allele to the offspring via their gametes.
A child who inherits the b allele from both parents will have the genotype bb and will have blue eyes, as there is no dominant allele present.

Examiner tips

  • Use the exact terms dominant, recessive, heterozygous, phenotype.
  • Show the logic of dominance masking the recessive allele.
  • Explain the probability of passing the b allele from each parent.
  • State that the child’s genotype bb gives the blue eye phenotype.

Common mistakes

  • Using ‘homozygous’ instead of ‘heterozygous’ for the parents.
  • Failing to mention that the brown eye phenotype is due to dominance.
  • Not linking the genotype bb to the blue eye phenotype.

Mark scheme (4 marks)

  1. Both parents must be heterozygous (Bb), carrying one dominant and one recessive allele
  2. The brown eye phenotype is expressed because B (brown) is dominant and masks the recessive b allele
  3. Each parent can pass either the B or the b allele to the offspring via their gametes
  4. A child who inherits the b allele from both parents will have the genotype bb and will have blue eyes, as there is no dominant allele present

Key terms in this question

dominant · recessive · heterozygous · phenotype

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