CodominanceSpec 3.21B
In short
Codominance is when both alleles of a heterozygous organism are expressed in the phenotype. In some cattle, the alleles for red hair (R) and white hair (W) are codominant, so an RW animal has both red and white hairs and is called roan. Crossing two roan cattle gives a 1 red : 2 roan : 1 white ratio.
Sometimes neither allele is dominant. Codominance is when both alleles are expressed in the phenotype of a heterozygous organism.
An example is coat colour in some cattle. The allele for red hair (R) and the allele for white hair (W) are codominant. Both are written as capital letters because neither is dominant. An animal with one of each, RW, has both red hairs and white hairs. It is called roan.
| Genotype | Phenotype |
|---|---|
| RR | Red |
| WW | White |
| RW | Roan (red hairs and white hairs) |
A cross with codominant alleles
A red bull (RR) is crossed with a white cow (WW). What are the genotype and phenotype of the calves?
- Gametes from the bull: all R. Gametes from the cow: all W.
- Every calf receives one of each allele, so every calf is RW.
- Both alleles are expressed, so every calf has red hairs and white hairs.
Answer: All calves are RW and roan.
If two roan cattle (RW × RW) are crossed, the offspring genotypes are 1 RR : 2 RW : 1 WW. Because the heterozygote has its own phenotype, the phenotype ratio is also 1 red : 2 roan : 1 white, not 3 : 1.
The ABO blood groups are another example. The allele for blood group A and the allele for blood group B are codominant, so a person who inherits one of each has blood group AB.
Codominance is not blending. In a roan animal, red hairs and white hairs are both there, not pink hairs.
Define codominance as both alleles being expressed in the phenotype of a heterozygous organism.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026