Codominance occurs when two versions, or “alleles,” of the same gene are present in a living thing, and both are expressed. Instead of one trait being dominant over the other, both traits appear.
Codominance is easy to spot in plants and animals that have more than one pigment color. Spotted cows and flowers with petals of two different colors are examples of codominance, for example.
Codominance also occurs in some less visible traits, such as blood type. The A and B alleles for blood type can both be expressed at the same time, resulting in type AB blood.
In genetics, “dominant” genes are those that are always expressed if they are found in an organism. Dominant genes may be expressed as co-dominant – where two different traits are both expressed alongside each other – or as dominant/recessive, where the presence of a dominant gene completely masks the presence of a recessive gene.
Examples of Codominance
When a chicken with white feathers breeds with a chicken with black feathers, the result is an offspring chicken that grows up to have both black and white feathers.
Likewise, when a red cattle breeds with a red cattle, the resulting offspring may show both red and white hairs, resulting in a mixed coat pattern called “roan.”
Rhododendrons and other flowers may also exhibit codominance.
In the case of rhododendrons, the crossing of a red and white flower may yield a flower that has both red and white patches.
Many flowers show similar patterns of codominance, where both of the parental flower colors show up in different parts of the plant.
An example of codominance that occurs in humans is that of blood type.
There are three different versions of the gene for proteins that appear on the outside of our blood cells and help our body to identify the cells as their own. These alleles are A, B, and O.
The “O” allele actually does not code for any protein at all, so people with the “O” trait lack both A and B proteins.
The A and B proteins, on the other hand, code for two different proteins. These proteins, like different colors in a flower, can appear together.
Someone who inherits an A allele from one parent and a B allele from the other will express both proteins in a codominant fashion, resulting in an AB blood type.
The “O” trait, on the other hand, is a good example of a dominant/recessive relationship: if either A or B is expressed, the “O” trait is not expressed.
This chart below illustrates how codominance can occur between A and B traits, while a dominant/recessive relationship exists between those traits and the O trait: