Mendel and Dominance
One of Mendel’s contributions to genetics is the concept of dominance, the idea that an individual that possesses two different alleles for a trait, but only one these alleles is expressed in the phenotype, leading to the moniker ‘dominant’.
When biologists began to apply Mendel’s principles, they soon noticed that there were many characteristics did not exhibit a clear form of dominance. It became clear that many of the interactions between alleles were far more complex than a simple dominant-recessive relationship. One type of such complex interaction, is known as codominance.
Codominance in genetics is a type of interaction between alleles. This denotes a situation where the phenotype of a heterozygote is not an expression of one of the two alleles, nor the intermediate between the phenotypes of the homozygotes (termed incomplete dominance). When a characteristic is expressed as a result of codominant alleles, the phenotype simultaneously expresses the phenotypes of both homozygotes.
A Human Example: MN Blood Type
Besides the well-known ABO and Rh blood types, there is another one called the MN blood type. Since foreign MN antigens do not elicit a strong immune reaction, the MN blood type is not often considered in blood transfusions, resulting in the MN blood type being less known.
The M and N alleles code for M and N antigens respectively. Homozygotes for either the M or N allele will only express the M or N antigen in their blood, and are attributed the M or N blood type respectively. Heterozygotes, however, exhibit codominance. This means that they express both the M and the N allele, which results in the expression of both the M and N antigens. The blood type given to these individuals is MN.
Level of Phenotype and Cystic Fibrosis
It is important to realize that the type of dominance exhibited by a trait depends on the level of the phenotype that is being considered. To clarify this, the expression of cystic fibrosis will be briefly explained. Cystic fibrosis is a genetic disease, generally considered recessive. Affected individuals produce thick, sticky mucus which blocks the airways and digestive paths, which leads to respiratory and digestive problems.
The gene responsible for this disorder resides on the long arm of the seventh chromosome. It produces the protein CFTR, which regulates the movement of chloride ions in and out the cells. Patients with cystic fibrosis posses a mutated form of this protein, causing chloride ions to build up in the cells, resulting in the production of mucus. The expression of the two alleles (normal and mutated) of the CFTR gene can be considered on two levels:
- The molecular level: Heterozygotes (one normal and one mutant CFTR allele) produce both normal and defective CFTR proteins, so at this level, the two alleles are codominant.
- The physiological level: Heterozygotes do produce sufficient CFTR to prevent mucus prevention, so at the level of the person, the mutated CFTR allele seems to be recessive.
- Maynard Smith, J. (1999) Evolutionary Genetics. Second Edition. Oxford University Press.
- Pierce, B.A. (2002) Genetics: A Conceptual Approach. First edition. W.H.Freeman Publishing.