Sometimes both the alleles of genes in a heterozygote lack the dominant and recessive relationship, that is, each allele is capable of some degree of phenotypic expression. In a sense, co-dominance is no dominance at all, the heterozygotes showing the phenotypes of both homozygotes. Hence heterozygote genotype gives rise to a phenotype, distinctly different from either of the homozygous genotypes of parent.

Example: The cote colour of the short horn breed of cattle represents a classical example of co-dominance. When cattle of red coat (C^RC^R) are crossed with cattle of white coat (C^wC^w), the F1 heterozygote is found to posses roan coat (C^RC^w). In roan coat the red and white hair occurs in definite patches but no hair a intermediate colour of red and white. In such a case the F2 phenotypic ratio and genotypic ratio are the same as follows: