Codominance is a phenomenon in genetics where two alleles (different versions of the same gene) are expressed to an equal degree within an organism. In codominance, both alleles are simultaneously expressed in the heterozygote, resulting in a phenotype that shows both traits. This is different from incomplete dominance, where the heterozygote phenotype is intermediate between the two homozygote phenotypes.
An example of codominance is the MN blood groups of humans, where an M marker and a slightly different N marker are both produced on the surface of red blood cells. Another example is the white-spotted red flower in plants and the black-and-white-coated mammals.
Mendels rules can still be used to predict inheritance of codominant alleles. Codominance can occur because both alleles of a gene are dominant, and the traits are equally expressed.
