1.5 Co-dominance Co-dominance is the situation where the heterozygote exhibits a phenotype that is intermediate between the two homozygous forms. When both genes in a heterozygous pairing are equally expressed, they are termed co-dominant genes.
An example of co-dominance is provided by the MN blood group system in humans, which is controlled by a single gene. One allele of the MN blood group gene results in the production of protein M on the red blood cell membrane while the other allele results in the formation of protein N.
Since neither allele is recessive, capital letters are used for both alleles: M represents the allele for the production of protein M. N represents the allele for the production of protein N.