Explain why a plant with the genotype Gg has a green phenotype rather than a yellow phenotype, given that G represents the allele for green colour and g represents the allele for yellow colour.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In pea plants, flower colour is controlled by a single gene. The allele for green (G) is dominant and the allele for yellow (g) is recessive.
Model answer (4 marks)
The plant is heterozygous, having one dominant allele (G) and one recessive allele (g). The dominant allele (G) is always expressed when present. The recessive allele (g) is only expressed in the absence of the dominant allele or when two copies are present. Therefore the green phenotype is expressed because at least one dominant allele is present.
Examiner tips
- Use the term ‘heterozygous’ to show understanding of genotype.
- Explain the dominance relationship before stating the phenotype.
- Mention that the recessive allele is masked by the dominant allele.
- Keep the answer concise and use correct terminology.”]
Mark scheme (4 marks)
- The plant is heterozygous, having one dominant allele (G) and one recessive allele (g)
- The dominant allele (G) is always expressed when present
- The recessive allele (g) is only expressed in the absence of the dominant allele / when two copies are present
- Therefore the green phenotype is expressed because at least one dominant allele is present
Key terms in this question
Related
- All Eduqas GCSE Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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