The codominance or codominant inheritance may be defined as equal force between alleles. If in incomplete dominance we can speak of a genetic dosage effect (AA> Aa> aa), in codominance we can say that we observe the joint manifestation of two products for the same character in the same individual, and with the same force.
One of the reasons that allowed Gregor Mendel to analyze in a simple way the inheritance patterns observed by him is that the characters under study were of complete dominance.
An example of codominance: Hybrid Camellia, pink and white (Camellia cultivar Rhododendron sp., Fam. Ericaceae). Photo taken in Japan. Darwin cruz, via Wikimedia Commons That is, it was enough that at least one dominant allele (A _) was present for the trait with the associated phenotype to be expressed; the other (a), receding in its manifestation and seemed to hide.
That is why, in these “classic” or Mendelian cases, the AA and Aa genotypes are phenotypically manifested in the same way (A completely dominates aa).
But this is not always the case, and for monogenic traits (defined by a single gene) we can find two exceptions that can sometimes be confused: incomplete dominance and codominance.
In the first, the Aa heterozygote manifests a phenotype intermediate to that of the AA and aa homozygotes; in the second, which is the one we are dealing with here, the heterozygote manifests the two alleles, A and a, with the same force, since in reality neither is recessive on the other.
Codominance example. Blood groups according to the ABO system
In order to finish understanding codominance, understood as equal strength between alleles, it is useful to define incomplete dominance. The first thing to clarify is that both refer to relationships between alleles of the same gene (and the same locus) and not to relationships or gene interactions between genes of different loci.
The other thing is that incomplete dominance manifests as a phenotype product of the dose effect of the product encoded by the gene under analysis.
Take a hypothetical case of a monogenic trait in which an R gene, which codes for a monomeric enzyme, gives rise to a color (or pigment) compound. The recessive homozygote for that gene (rr) will obviously lack that color because it does not give rise to the enzyme that produces the respective pigment.
Both the homozygous dominant RR and the heterozygous Rr will show color, but in a different way: the heterozygote will be more dilute since it will present half the dose of the enzyme responsible for producing the pigment.
It should be understood, however, that genetic analysis is sometimes more complicated than the simple examples provided here, and that different authors interpret the same phenomenon differently.
It is possible, therefore, that in dihybrid crosses (or even with more genes from different loci) the analyzed phenotypes may appear in proportions that are similar to those of a monohybrid cross.
Only rigorous and formal genetic analysis can allow the researcher to conclude how many genes are involved in the manifestation of a character.
Historically, however, the terms codominance and incomplete dominance were used to define allelic interactions (genes from the same locus), while those that refer to gene interactions from different loci, or gene interactions per se, are all analyzed. as epistatic interactions.
The analysis of the interactions of different genes (of different loci) that lead to the manifestation of the same character is called epistasis analysis - which is basically responsible for all genetic analysis.
References
- Brooker, RJ (2017). Genetics: Analysis and Principles. McGraw-Hill Higher Education, New York, NY, USA.
- Goodenough, UW (1984) Genetics. WB Saunders Co. Ltd, Pkiladelphia, PA, USA.
- Griffiths, AJF, Wessler, R., Carroll, SB, Doebley, J. (2015). An Introduction to Genetic Analysis (11 th ed.). New York: WH Freeman, New York, NY, USA.
- White, D., Rabago-Smith, M. (2011). Genotype-phenotype associations and human eye color. Journal of Human Genetics, 56: 5-7.
- Xie, J., Qureshi, AA, Li., Y., Han, J. (2010) ABO blood group and incidence of skin cancer. PLoS ONE, 5: e11972.