Co-dominance

Red and white pom pom.dahlia
A pom pom.dahlia showing co-dominance. By User:Darwinek (Own work) [ CC BY-SA 3.0 or GFDL], via Wikimedia Commons

Co-dominance is a type of non-Mendelian inheritance pattern that finds the traits expressed by the alleles to be equal in the phenotype. There is neither a complete dominance or incomplete dominance of one trait over the other for that given characteristic. Co-dominance would show both alleles equally instead of a blending of the traits as is seen in incomplete dominance.

In the case of co-dominance, the heterozygous individual expresses both alleles equally. There is no mixing or blending involved and each is distinct and equally shown in the phenotype of the individual. Neither trait masks the other like in simple or complete dominance, either.

Many times, co-dominance is linked with a characteristic that has multiple alleles. That means there is more than just two alleles that code for the trait. Some traits have three possible alleles that may combine and some traits have even more than that. Oftentimes, one of those alleles will be recessive and the other two will be co-dominant. This gives the trait the ability to follow the Mendelian Laws of heredity with simple or complete dominance or, alternatively, to have a situation where co-dominance comes into play.

Examples

One example of co-dominance in humans is the AB blood type. Red blood cells have antigens on them that are designed to fight off other foreign blood types, which is why only certain types of blood can be used for blood transfusions based on the recipient's own blood type. A type blood cells have one kind of antigen, while the B type blood cells have a different type. Normally, these antigens would signal that they are a foreign blood type to the body and would be attacked by the immune system. People with AB blood types have both antigens naturally in their systems, so their immune system will not attack those blood cells.

This makes people with the AB blood type "universal recipients" due to the co-dominance displayed by their AB blood type. The A type does not mask the B type and vice versa. Therefore, both the A antigen and B antigen are equally expressed in a display of co-dominance.