What is codominance?

When both alleles are expressed in the same phenotype

Codominance is when 2 dominant alleles both show in the same phenotype, for example for human blood types there are 3 possible alleles: IA, IB, and IO. IA and IB are both dominant over IO, but are not dominant over each other, therefore if you have one IA allele and one IB allele your blood type will be AB rather than just A or B. This is codominance as both alleles are expressed in your phenotype.

Another example would be if a rabbit had 2 dominant genes: one for black fur and one for white fur, and had black and white speckled fur as a result of expressing both genes.

For reference phenotypes are the visible characteristics of an organism which occur as a result of its genes.