Codominance is a type of inheritance in which both alleles of a gene are expressed equally in the phenotype of the organism. This means that neither allele is dominant or recessive, and both traits are expressed simultaneously. Codominance is different from incomplete dominance, where the heterozygous phenotype is a blend of the two homozygous phenotypes.

Examples of codominance in humans include the MN blood group system and the ABO blood group system. In the MN blood group system, individuals who are homozygous for the M allele have a surface molecule called the M antigen on their red blood cells, while those homozygous for the N allele have the N antigen on their red blood cells. Heterozygotes carry both antigens. In the ABO blood group system, there are three alleles: A, B, and O. Individuals with the A allele have A antigens on their red blood cells, those with the B allele have B antigens, and those with the O allele have neither A nor B antigens. Individuals with both the A and B alleles have both A and B antigens, making them AB blood type.

Codominance is also seen in animals and plants. Examples include speckled chickens, which have alleles for both black and white feathers, and roan cattle, which express alleles for both red hair and white hair. In plants, codominance is exemplified by rhododendrons with simultaneous expression of red and white genes for flower color, which display flowers with both red and white petals.

Understanding codominance is important in genetics, as it helps explain the inheritance of traits and the expression of genes in organisms.