A plant species can produce flowers that are either red, pink or white. A student crosses two pink-flowered plants and obtains offspring in the ratio 1 red : 2 pink : 1 white. Explain how this ratio arises, and state what this pattern of inheritance is called.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The pink flowers are heterozygous (Rr) with one red allele (R) and one white allele (r). 2. The cross is Rr × Rr. 3. The Punnett square gives 1 RR : 2 Rr : 1 rr, i.e. 1 red : 2 pink : 1 white. 4. In the heterozygotes neither allele is completely dominant; both are expressed equally, giving the intermediate pink phenotype – a case of incomplete dominance (codominance).
Examiner tips
- Use the exact terms ‘heterozygous’, ‘incomplete dominance’ and ‘pink is intermediate’.
- Show the Punnett square or state the genotypic ratio clearly.
- Explain the phenotype–genotype link in the heterozygote.
Common mistakes
- Writing ‘dominant’ instead of ‘incomplete dominance’.
- Assuming pink is a separate allele rather than an intermediate phenotype.
Mark scheme (4 marks)
- Names the pattern of inheritance as incomplete (or partial) dominance / codominance of both alleles over the other (accept co-dominance only if the student explicitly states both alleles are expressed equally — do not accept dominance alone)
- States that pink-flowered plants are heterozygous (one allele for red, one for white)
- Correctly identifies the cross as heterozygous × heterozygous (or gives correct parental genotypes), producing offspring in genotypic ratio 1 homozygous red : 2 heterozygous : 1 homozygous white
- Links phenotype to genotype by explaining that in heterozygotes neither allele fully masks the other, so an intermediate (pink) phenotype results from the expression of both alleles
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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