# Incomplete dominance and codominance

In incomplete dominance the heterozygote looks like a blend of the two homozygotes. In codominance both alleles show fully and separately.

Biology (university) · Part 07: Genetics and inheritance · Card 4 of 8 · Last updated 2026-10-19

## Analogy

Mixing red and white paint (blend) versus a red and white striped shirt (both).

## In practice

Snapdragons: red x white gives pink. Human AB blood type: both A and B markers are present.

## Practice

### Flashcards

- **What is codominance?** Both alleles are fully expressed in the heterozygote, as in AB blood.

## Questions people ask

### How is codominance different from incomplete dominance?

In incomplete dominance the heterozygote shows a new, in-between version, like pink flowers from red and white parents. In codominance both alleles are expressed separately in the same individual, such as a roan cow with red and white hairs, or AB blood with both A and B markers.

### How does ABO blood type inheritance work?

Three alleles exist: I^A, I^B and i. I^A and I^B are codominant, and both are dominant to i. So I^A i gives type A, I^B i gives type B, I^A I^B gives type AB, and ii gives type O. Parents with A and B can have a child with O.

## Related concepts

- [Allele](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-1)
- [Law of segregation](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2)
- [Law of independent assortment](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-3)
- [Sex-linked inheritance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-5)

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