Explain why natural selection acts on phenotype rather than genotype, and outline the consequence of this for recessive alleles in a diploid population.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Natural selection acts on phenotype because it is the expressed physical or biochemical characteristics that determine an organism’s survival and reproductive success, not the DNA sequence itself.
Genotype influences phenotype, but the same phenotype can arise from different genotypes (e.g. a homozygous dominant and a heterozygous individual may be phenotypically identical), so selection cannot distinguish between them.
A recessive allele in the heterozygous state is not expressed and therefore is not exposed to natural selection; it is effectively hidden from selection.
Consequently, harmful recessive alleles can persist in a population at low frequency because they are maintained in heterozygous carriers and only experience selection when they appear in the homozygous recessive genotype.
Genotype influences phenotype, but the same phenotype can arise from different genotypes (e.g. a homozygous dominant and a heterozygous individual may be phenotypically identical), so selection cannot distinguish between them.
A recessive allele in the heterozygous state is not expressed and therefore is not exposed to natural selection; it is effectively hidden from selection.
Consequently, harmful recessive alleles can persist in a population at low frequency because they are maintained in heterozygous carriers and only experience selection when they appear in the homozygous recessive genotype.
Examiner tips
- Use the word ‘phenotype’ first, then explain why it matters for survival/reproduction. Mention that different genotypes can give the same phenotype. State that recessive alleles are hidden in heterozygotes. Show the consequence: persistence of recessive alleles in the population.
Common mistakes
- Confusing genotype with phenotype as the target of selection. Forgetting that heterozygotes hide recessive alleles. Over‑generalising that all recessive alleles are always harmful or always neutral.
Mark scheme (4 marks)
- Natural selection acts on phenotype because it is the expressed physical/biochemical characteristics that determine an organism's survival and reproductive success, not the DNA sequence itself.
- Genotype influences phenotype, but the same phenotype can arise from different genotypes (e.g. homozygous dominant and heterozygous individuals may be phenotypically identical), so selection cannot distinguish between them.
- A recessive allele in the heterozygous state is not expressed and therefore not exposed to selection; it is effectively hidden from natural selection.
- Consequently, harmful recessive alleles can persist in a population at low frequency because they are maintained in heterozygous carriers and only experience selection when they appear in the homozygous recessive genotype.
Key terms in this question
phenotype · genotype · natural selection · recessive allele · diploid
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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