Codominance is one of the Non-mendelian inheritance patterns in genetics i.e. does not follow Mendel's principles of inheritance. Codominance is a phenomenon whereby an allele is neither dominant nor recessive to another in a gene i.e. one allele is not phenotypically expressed over the other. Hence, both alleles are simultaneously expressed in their heterozygous state.
An example of codominance is the roan coat color in horses where the colored (B) and white (W) alleles are both dominant in the coat color gene, hence, the horse expresses both black and white coat phenotypes known as ROAN (BW). The black and white alleles are said to be CODOMINANT.
