In guinea pigs, coat length is controlled by a single gene. The allele for short coat (S) is dominant over the allele for long coat (s). Two guinea pigs, both with short coats, are crossed. Among their offspring, some have long coats. Explain how two short-coated parents can produce offspring with a long coat.

Pearson Edexcel International GCSE Biology (4BI1) — 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 (Ss). Each parent can produce gametes carrying either the S or the s allele. When fertilisation occurs there is a chance that two s gametes combine, giving a ss genotype. The ss offspring is homozygous recessive, so the long‑coat phenotype is expressed.

Examiner tips

  • State that both parents are heterozygous (Ss).
  • Explain that each parent produces S and s gametes.
  • Show that two s gametes can combine to give ss.
  • Mention that ss gives the long‑coat phenotype.

Common mistakes

  • Assuming one parent is homozygous dominant (SS).
  • Failing to explain the role of recessive allele s.

Mark scheme (4 marks)

  1. Both parents are heterozygous (Ss)
  2. Each parent produces two types of gamete: gametes carrying S and gametes carrying s
  3. There is a chance that two s gametes combine at fertilisation to give the genotype ss
  4. The ss offspring is homozygous recessive so the long coat phenotype is expressed

Key terms in this question

allele · dominant

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