# Explain how multiple alleles at a single gene locus can produce more than two phenotypes in a population, using the ABO blood group system as an example.

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

## Mark scheme (4 marks)

1. A gene can exist in more than two allelic forms (multiple alleles) in a population, even though each individual carries only two alleles.
2. In the ABO system there are three alleles: I^A, I^B, and i (or equivalent correct notation), controlling blood group.
3. I^A and I^B are both dominant over i and codominant with each other, so different combinations of two alleles from three possible alleles produce four distinct phenotypes: A, B, AB, and O.
4. The greater number of phenotypes arises because different combinations (genotypes) of the three alleles — such as I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B, and ii — produce distinct phenotypic outcomes that would not be possible with only two alleles.

## Key terms

- [multiple alleles](https://www.gradenine.co.uk/glossary/multiple-alleles)
- [ABO blood group](https://www.gradenine.co.uk/glossary/abo-blood-group)
- [locus](https://www.gradenine.co.uk/glossary/locus)
- [phenotype](https://www.gradenine.co.uk/glossary/phenotype)

## Related

- [Revision notes for IB DP Biology Standard 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-how-multiple-alleles-at-a-efa13254) · Published by Druglandscape Ltd.