In humans, the allele for freckles (A) is dominant over the allele for no freckles (a). Two parents who both have freckles have a child with no freckles. Explain how this is possible, using the terms genotype and phenotype in your answer.

Eduqas GCSE Biology — 2.3.2 Inheritance · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both parents are heterozygous (Aa) – they carry one dominant allele for freckles (A) and one recessive allele for no freckles (a). Each parent can pass either allele to their child. The child inherits the recessive allele (a) from both parents, giving the genotype aa. Because the recessive allele is only expressed when no dominant allele is present, the child’s phenotype is no freckles. The parents still show freckles (phenotype) because the dominant allele (A) masks the recessive allele (a) in their heterozygous genotype.

Examiner tips

  • Use the terms genotype and phenotype explicitly.
  • Show the parents’ heterozygous genotype (Aa) and the child’s homozygous recessive genotype (aa).
  • Explain why the recessive phenotype appears only when both alleles are recessive.

Mark scheme (4 marks)

  1. Both parents must carry the recessive allele for no freckles (a), i.e. both are heterozygous (Aa)
  2. The child inherits the recessive allele from each parent, giving the homozygous recessive genotype (aa)
  3. Because the allele for no freckles is recessive, it is only expressed in the phenotype when no dominant allele is present
  4. The parents have freckles as their phenotype despite carrying the recessive allele because the dominant allele (A) masks the recessive allele (a) in heterozygous individuals

Key terms in this question

allele · dominant · genotype · phenotype

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