Explain how a child can inherit a genetic disorder from two parents who do not show any symptoms of that disorder.

WJEC GCSE Biology (Wales) — 2.3.2 Inheritance · Explain · 4 marks · View as Markdown

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

Some genetic disorders are caused by recessive alleles. A couple, neither of whom has the disorder, are surprised to find that their child has been diagnosed with a recessive genetic disorder.

Model answer (4 marks)

Both parents carry the recessive allele for the disorder.
Parents do not show symptoms because they also carry the dominant allele.
Each parent can pass either the dominant or recessive allele to the child.
The child inherits two recessive alleles (homozygous recessive) and therefore shows the disorder.

Examiner tips

  • Use the word ‘carry’ to show heterozygosity. Explain why the parents are asymptomatic (dominant allele present). Show the 50:50 chance of passing each allele. State that the child is homozygous recessive to justify the disorder.

Common mistakes

  • Saying the parents are ‘affected’ instead of ‘carriers’. Forgetting to mention that the parents are heterozygous. Not explaining why the child shows the disorder (homozygous recessive).

Mark scheme (4 marks)

  1. Both parents carry the recessive allele for the disorder
  2. Parents do not show symptoms because they also carry the dominant allele
  3. Each parent can pass either the dominant or recessive allele to the child
  4. Child inherits two recessive alleles (homozygous recessive) and therefore shows the disorder

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