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Dominance relations

In diploid organisms, the dominance relationship between two alleles determines which allele is expressed.

All alleles can be grouped into one of three categories:

  • Dominant alleles are expressed in both homozygous (e.g. AA) and heterozygous (e.g. Aa) individuals.
  • Recessive alleles are only expressed in homozygous individuals.
  • Codominant alleles are partially expressed in heterozygous individuals.

Dominance relationships have a specific notation. A dominant allele is indicated by an upper case letter. A recessive allele is indicated by a lower case letter. Codominance is indicated by superscripts.

Example of the different dominance relationships.
Example of the different dominance relationships.

Codominance describes a form of dominance relationship in which both alleles of a gene contribute to the phenotype.

Codominance is common at the molecular level. Typically, both alleles are transcribed into proteins.

Usually one protein will be more functional or produced in larger quantities, and so once allele is dominant, even though both alleles are expressed at the molecular level. Sometimes the result of both alleles is visible in the phenotype, as in the flower below.

Codominance should not be confused with epistasis. Epistasis is the interaction of two different genes, while codominance is the interaction of two alleles of the same gene.

The red and white alleles of this dahlia are both visible in the phenotype. This is an example of codominance.
The red and white alleles of this dahlia are both visible in the phenotype. This is an example of codominance.

The ABO blood group system is a good example of different allele dominance relationships.

Blood can be grouped into four types: A, B, AB or O. This grouping refers to the type of antigen expressed by red blood cells.

The antigens are expressed by three alleles:

  • $$\text{I}^\text{A}$$ codes for antigen A.
  • $$\text{I}^\text{B}$$ codes for antigen B.
  • $$\text{I}^\text{O}$$ does not code for an antigen.

$$\text{I}^\text{A}$$ and $$\text{I}^\text{B}$$ are both dominant to $$\text{I}^\text{O}$$. $$\text{I}^\text{O}$$ is recessive. If both $$\text{I}^\text{A}$$ and $$\text{I}^\text{B}$$ are present, they are codominant.

These allele interactions create the following genotype-phenotype table:

Genotype IAIA or IA IO IBIB or IB IO IAIB IOIO
Antigens A antigen B antigen A and B antigens None
Phenotype