A cat breeder is studying eye colour in Siamese cats. Blue eye colour is controlled by a dominant allele (B) and green eye colour is controlled by a recessive allele (b). One parent cat is homozygous dominant and the other parent cat is homozygous recessive. Explain what the genotypes and phenotypes of all offspring from this cross would be, and explain why all the offspring have the same phenotype.

Edexcel GCSE Biology (1BI0) — 3.4 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 BB (homo‑dominant) and bb (homo‑recessive). Each parent contributes one allele, so all offspring receive one B and one b, giving the genotype Bb. The phenotype of Bb is blue eyes, because B is dominant and masks the recessive b. Therefore every kitten has blue eyes.

Examiner tips

  • State both parental genotypes first; then give offspring genotype; then phenotype; finally explain dominance.
  • Use the word ‘dominant’ and ‘recessive’ correctly; mention ‘masking’ or ‘expressed over’ to show understanding.

Common mistakes

  • Confusing blue with green as dominant; writing bb as the offspring genotype; omitting the explanation of dominance.

Mark scheme (4 marks)

  1. States that both parent genotypes are BB and bb respectively (or correctly identifies both parental gametes as B and b)
  2. States that all offspring have the genotype Bb (heterozygous)
  3. States that all offspring have blue eyes (dominant phenotype)
  4. Explains that all offspring show the dominant phenotype because B is dominant and masks / is expressed over the recessive allele b

Key terms in this question

homozygous · dominant · recessive · genotype · phenotype · allele

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