What is codominance?

Codominance occurs when two different alleles are equally expressed in an organism's phenotype. Unlike typical dominant-recessive gene relationships, both alleles in codominance share equal dominance.

This means that both alleles are visible in the organism's observable characteristics, leading to a phenotype that is a mixture of both alleles rather than just one.

How to show codominance in genetic diagrams:

• An uppercase letter is used to signify the gene (e.g. C for the colour of a particular trait in an organism).
• Superscript uppercase letters indicate the alleles (e.g. C^R and C^W for red or white colouration).
• Avoid lowercase letters to prevent the implication of recessiveness.

An example of codominance in flower colour

The flower colour in snapdragons demonstrates codominance.

The alleles responsible for snapdragon flower colour are:

• C^R - Responsible for a high production of red pigment.
• C^W - Leads to a low production of red pigment.

There are three potential flower colours based on these alleles:

In pink snapdragons, the blended colour results from the combined influence of both alleles: the C^R allele generates a certain amount of red pigment, while the C^W allele contributes minimally, creating a pink hue.

Worked example - Identifying offspring phenotypes with codominant alleles

Shorthorn cattle may have red, white, or roan coloured coats. The alleles C^R and C^W are codominant and produce the roan coat colour.

Identify the phenotypic ratio in the F₂ generation in a cross between a cow with a white coat and a bull with a red coat.

Step 1: State the parents’ genotypes and gametes

as the parents are not roan coloured, they must be homozygous for the coat colour alleles

white coat parent genotype - C^WC^W, gametes C^W

red coat parent genotype - C^RC^R, gametes C^R

Step 2: Perform a cross to identify the genotypes of the F₁ generation

Step 3: Perform a cross between F₁ offspring to identify the genotypes of F₂ generation

Step 4: State the ratio of F₂ offspring phenotypes

F₂ offspring genotypes: 1 C^RC^R, 2 C^RC^W, 1 C^WC^W

as the alleles are codominant, when they are heterozygous they produce a blended roan phenotype

the ratio of F₂ offspring phenotypes is 1 red to 2 roan to 1 white