A plant species can produce flowers that are either red, pink, or white. A student crossed two pink-flowered plants and obtained offspring in an approximate ratio of 1 red : 2 pink : 1 white. Explain how this ratio arises, naming the type of inheritance involved.

OCR A-Level Biology A (H420) — 6.2 Patterns of 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)

The pattern is due to incomplete dominance. Each pink plant is heterozygous (Rr) with one red allele (R) and one white allele (r). When two heterozygotes are crossed, the Punnett square gives:

   R  r
R  RR  Rr
r  Rr  rr

Thus the genotypic ratio is 1 RR : 2 Rr : 1 rr. The RR plants are red, the rr plants are white, and the Rr plants are pink because neither allele masks the other – both are expressed, giving an intermediate colour.

Examiner tips

  • State the inheritance type first (incomplete dominance).
  • Show the Punnett square or explain the 1:2:1 genotypic ratio.
  • Link genotype to phenotype – explain why heterozygotes are pink.

Mark scheme (4 marks)

  1. Identifies the type of inheritance as incomplete (incomplete dominance / codominance acceptable only if correctly applied to this phenotypic pattern)
  2. States that pink-flowered plants are heterozygous, carrying one allele for red and one allele for white
  3. Explains that crossing two heterozygotes produces offspring in the expected 1:2:1 genotypic ratio (homozygous red : heterozygous pink : homozygous white)
  4. Links genotype to phenotype by explaining that in heterozygotes neither allele masks the other, so both are expressed, producing the intermediate pink colour

Key terms in this question

ratio

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