A plant can produce either red flowers or white flowers. Red flower colour is controlled by a dominant allele (R). White flower colour is controlled by a recessive allele (r). Two plants that are heterozygous for flower colour are crossed. Explain why some of the offspring from this cross will have white flowers.

OCR GCSE Biology A: Gateway Science (J247) — B5.1 Inheritance · Explain · 4 marks · View as Markdown

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

Flower colour in a particular plant species is controlled by a single gene with two alleles. Red flower colour is dominant over white flower colour.

Model answer (4 marks)

Both parents are heterozygous Rr.
Each parent can pass either R or r to the offspring.
Some gametes will carry r from both parents, producing rr offspring.
Only rr plants express the white phenotype, so these offspring have white flowers.

Examiner tips

  • State the parents’ genotypes first. Explain the possible gametes. Show how rr gives white flowers. Use the word "only" to emphasise the recessive condition.

Common mistakes

  • Saying both parents are homozygous. Forgetting to mention that white appears only when both alleles are r. Using vague terms like "some" without showing the genetic reasoning.

Mark scheme (4 marks)

  1. Both parent plants have the genotype Rr (heterozygous)
  2. Each parent can pass on either the R allele or the r allele to offspring
  3. Some offspring will inherit the r allele from both parents, giving the genotype rr
  4. The white phenotype is only expressed when two copies of the recessive allele are present (rr), so rr offspring have white flowers

Key terms in this question

allele · dominant · recessive · heterozygous

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