# Explain how multiple alleles and codominance together can produce more phenotypes in a population than would be possible with a single gene having only two alleles showing simple dominance.

> IB DP Biology Higher Level (2023 syllabus) — D3.2 Inheritance · Explain · 4 marks

> The ABO blood group system in humans is controlled by a single gene (the I gene) that exists as three alleles: I^A, I^B, and i. I^A and I^B are codominant to each other, and both are dominant over i.

## Mark scheme (4 marks)

1. Multiple alleles means more than two allelic forms of a gene exist in the population, increasing the number of possible genotypic combinations.
2. With simple dominance and only two alleles, only two phenotypes are possible, because heterozygotes express the same phenotype as the dominant homozygote.
3. Codominance means that when both codominant alleles are present in a heterozygote, both are expressed simultaneously, producing a distinct/additional phenotype not seen in either homozygote.
4. The combination of multiple alleles and codominance therefore produces four phenotypes (A, B, AB, O) from three alleles, which is more than the maximum of two phenotypes possible with two alleles and simple dominance.

## Key terms

- [multiple alleles](https://www.gradenine.co.uk/glossary/multiple-alleles)
- [codominance](https://www.gradenine.co.uk/glossary/codominance)
- [simple dominance](https://www.gradenine.co.uk/glossary/simple-dominance)
- [phenotype](https://www.gradenine.co.uk/glossary/phenotype)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-multiple-alleles-and-codominance-bf8ed144) · Published by Druglandscape Ltd.