# Explain how codominance differs from complete dominance, using an example from genetics to support your answer.

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

## Mark scheme (4 marks)

1. In complete dominance, one allele masks the expression of the other allele entirely, so the heterozygote has the same phenotype as the dominant homozygote.
2. In codominance, both alleles are expressed simultaneously in the heterozygote, producing a phenotype distinct from either homozygote.
3. A valid example of codominance is given, such as ABO blood groups where the I^A and I^B alleles are codominant, producing blood group AB in a heterozygote (I^A I^B) with both A and B antigens present on red blood cells.
4. Codominant alleles are represented by two uppercase letters (or superscripts), reflecting that neither allele is dominant over the other, whereas in complete dominance uppercase (dominant) and lowercase (recessive) letters are used.

## Key terms

- [codominance](https://www.gradenine.co.uk/glossary/codominance)
- [complete dominance](https://www.gradenine.co.uk/glossary/complete-dominance)

## 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-codominance-differs-from-complete-5d7ebacd) · Published by Druglandscape Ltd.