Principles of Genetics and Breeding

3.1.5.2 Codominant alleles

In some situations the two alleles at a locus are co-dominant,i.e., both contribute equally to the phenotypic character of the heterozygote. This inheritance pattern is difficult to distinguish from the partial or incomplete dominance of a single allele, but fortunately the consequences, from the point of view of the breeder, are the same.

• Obviously, to produce 100% heterozygote phenotype offspring, matings must occur between two homozygotes.
• Alleles which lack dominant and recessive relationships may be called intermediate or codominant alleles. This means that each allele is capable of some degree of expression when in the heterozygous condition.
• Hence the heterozygous genotype gives rise to phenotype distinctly from either of the homozygous genotypes. Usually heterozygous phenotype resulting from codominance is intermediate in character between those produced by the homozygous genotypes, hence the erroneous concept of “blending”. The phenotypes may appear to be a “blend” in heterozygotes but the alleles maintain individual identities and will segregate from each other in the formation of gametes.
• Example: Blood group system in human beings. A is dominant to O, B is dominant to O, A and B are codominant and O is recessive to A and B.