# Explain why natural selection acts on phenotype rather than genotype, and outline the consequence of this for recessive alleles in a diploid population.

> IB DP Biology Higher Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks

## Mark scheme (4 marks)

1. 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.
2. 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.
3. A recessive allele in the heterozygous state is not expressed and therefore not exposed to selection; it is effectively hidden from natural selection.
4. 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

- [phenotype](https://www.gradenine.co.uk/glossary/phenotype)
- [genotype](https://www.gradenine.co.uk/glossary/genotype)
- [natural selection](https://www.gradenine.co.uk/glossary/natural-selection)
- [recessive allele](https://www.gradenine.co.uk/glossary/recessive-allele)
- [diploid](https://www.gradenine.co.uk/glossary/diploid)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-natural-selection-acts-on-54237a9c) · Published by Druglandscape Ltd.