A farmer has a herd of cattle. Some cattle have horns and some do not have horns. The allele for horns (H) is dominant over the allele for no horns (h). Two cattle that both have horns are crossed. Some of their offspring do not have horns. Explain how two cattle that both have horns can produce offspring without horns.

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).

Model answer (4 marks)

Both parents are heterozygous (Hh) and therefore carry the recessive allele for no horns.
The dominant allele (H) masks the recessive allele (h) in each parent, so both parents show the horned phenotype.
During fertilisation each parent contributes one allele to the offspring.
If the offspring receives the recessive allele from both parents (hh) it will not have horns.

Examiner tips

  • State that both parents are heterozygous (Hh).
  • Explain that H masks h in the parents but not when both alleles are h.
  • Show that each parent passes one allele and that hh gives the no‑horn phenotype.

Common mistakes

  • Claiming the parents are homozygous dominant (HH).
  • Forgetting to mention that the recessive allele is not expressed when H is present.
  • Not linking the hh genotype to the no‑horn phenotype.

Mark scheme (4 marks)

  1. Both parents must carry the recessive allele for no horns
  2. The recessive allele (h) is not expressed when the dominant allele (H) is present
  3. Each parent passes one allele to the offspring during sexual reproduction / fertilisation
  4. An offspring that inherits two recessive alleles (hh) will not have horns

Key terms in this question

dominant · allele

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