A scientist is studying petal colour in a species of flowering plant. Petal colour is controlled by a single gene. The allele for purple petals (P) is dominant over the allele for white petals (p). Two plants that are both heterozygous for petal colour are crossed. Explain what is meant by the terms dominant and heterozygous, and predict the expected genotypes and phenotypes of the offspring from this cross, using a Punnett square approach described in words.

OCR A-Level Biology B — Advancing Biology (H422) — 5.2 Inheritance and variation · Explain · 5 marks · View as Markdown

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

In a population of flowering plants, purple-petalled plants are more common than white-petalled plants. A scientist crosses two heterozygous purple-petalled plants to investigate how petal colour is inherited.

Model answer (5 marks)

Dominant means the allele is expressed when present, even if only one copy is present in a heterozygous individual. Heterozygous means the individual has two different alleles of a gene, for example Pp. When two heterozygous purple‑petalled plants (Pp × Pp) are crossed, the possible gametes are P and p from each parent. The Punnett square gives four genotype combinations: PP, Pp, Pp, pp. This corresponds to a 1:2:1 ratio of genotypes. Phenotypically, PP and both Pp offspring show purple petals because the dominant P allele is present. The pp offspring show white petals because they have two copies of the recessive allele and no dominant allele is present. Thus the expected phenotypic ratio is 3 purple : 1 white.

Examiner tips

  • Define dominant and heterozygous clearly; use the example Pp for heterozygous.
  • Show the four genotype combinations and state the 1:2:1 ratio.
  • Explain why PP and Pp give purple and pp gives white.
  • Mention the 3:1 phenotypic ratio to cover all marks.

Common mistakes

  • Confusing heterozygous with homozygous; writing PP as heterozygous.
  • Failing to explain why pp is white (missing recessive allele explanation).
  • Not showing the full Punnett square or the 1:2:1 genotype ratio.

Mark scheme (5 marks)

  1. Dominant means the allele is always expressed when present (even if only one copy is present / in a heterozygous individual)
  2. Heterozygous means the individual has two non-identical alleles of a gene (e.g. Pp)
  3. Correct identification of all four possible genotype combinations from the cross: PP, Pp, Pp, pp (accept 1PP : 2Pp : 1pp or equivalent ratio)
  4. Three offspring with purple petals (PP and Pp) and one offspring with white petals (pp) — accept 3:1 ratio stated or implied
  5. White-petalled offspring (pp) have white petals because they have two copies of the recessive allele / no dominant allele is present

Key terms in this question

dominant · heterozygous · allele · genotype · phenotype

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