A plant breeder crosses two true-breeding varieties of snapdragon. One variety has red flowers and the other has white flowers. All of the F1 offspring have pink flowers. When the F1 plants are allowed to self-fertilise, the F2 generation contains red, pink and white flowered plants. Explain why the F2 generation contains plants with three different flower colours, rather than showing the 3:1 ratio expected from a monohybrid cross involving complete dominance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Snapdragon flower colour is controlled by a single gene with two alleles. The allele for red pigment production is represented by C^R and the allele for no pigment production is represented by C^W. Each allele contributes to the phenotype independently.
Model answer (4 marks)
The two alleles are neither dominant nor recessive – they exhibit codominance (or incomplete dominance). The F1 plants are all heterozygous (C^R C^W) and therefore express both alleles, giving pink flowers. Self‑fertilised F1 plants produce the three genotypes: C^R C^R (red), C^R C^W (pink) and C^W C^W (white). Each genotype gives a distinct phenotype, so the F2 shows three colour classes rather than a 3:1 ratio.
Examiner tips
- State codominance/incomplete dominance first; mention F1 heterozygosity; list all three F2 genotypes; link each genotype to its phenotype
Common mistakes
- Confusing codominance with incomplete dominance; assuming complete dominance and expecting a 3:1 ratio; omitting the C^W C^W genotype
Mark scheme (4 marks)
- Identifies that neither allele is dominant over the other / codominance or incomplete dominance is occurring
- States that the F1 plants are all heterozygous (C^R C^W) and produce pink flowers because both alleles are expressed simultaneously
- Correctly identifies the three F2 genotypes produced from self-fertilisation of the F1
- Links each F2 genotype to its phenotype, explaining that the heterozygote produces a distinct (pink) phenotype giving three phenotype classes
Key terms in this question
true-breeding · self-fertilise · monohybrid cross · complete dominance
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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