Codominance

Codominance is a different kind of allelic interaction from incomplete dominance: instead of blending into an intermediate phenotype, both alleles of the heterozygote are expressed simultaneously and jointly, each producing its own distinct effect side by side rather than merging. Classic examples include the red-and-white flowers of Camellia, the inheritance of sickle-cell haemoglobin, and, most familiar to students, the human ABO blood group system, in which the IA and IB alleles of the I gene are codominant with each other (while both are dominant to the i allele) - an AB-blood-type individual expresses both the A and B antigens on their red cells at once, following Mendel's Law of Segregation just as any other allele pair does. Codominance has also been demonstrated directly in plants using protein-level techniques: when Gossypium hirsutum and Gossypium sturtianum, two cotton species with different seed-protein banding patterns on electrophoresis or chromatography, are hybridised, their F1 amphiploid hybrid shows an additive banding pattern containing both parental protein bands together, rather than just one or an average of the two. A codominant het …