Co-dominance vs. Incomplete dominant

Written by: Reptiles Breeding Enterprise

Unfortunately a very large proportion of people think that Incomplete dominant gnes are called co-dominant. "Intermediate" is the original term used by Gregor Mendel when he described his observations.

To our knowledge, there is no true co-dominant gene in Hognose snakes,  Ball pythons, Corn snakes or Leopard geckos as of the date of this article (January 2012). For this reason,  we now use an image editing program to highlight the major difference between these two terms. For visual clarity, we use the same mutation in both descriptions. In both cases it is the patternless gene, which is responsible for the phenotype of the Anacondas.

Incomplete dominant genes
In an heterozygous genotype (this means the genes that cause the look of an animal) it will display a phenotype (this is the look to the eyes of the beholder) that differs in its features and overall appearance from both genes. The phenotype is a mixed form. This is what we see when people talk about "Co-dom" mutations.

When the patternless gene is combined with the normal gene for this locus on the chromosome pair, the result will be a mixed phenotype. Neither the standard pattern nor the patternless effect will be pure visibly.


Co-dominant genes
In the heterozygous genotype both genes have "equal rights" to form the phenotype. The two genetic variants must be on the same locus on the chromosome pair and thus form a direct gene pair. Each of these two genes are easy to identify in the phenotype with their characteristics and full expression. They do not form a sort of mixture between the features.

In our particular case, this means the heterozygous form had normal patterned parts ([like as is typical for hognose snakes) and patternless areas (as we can see in the Supercondas). So you can see both traits together, but side by side.


Here are both phenotypes seen side by side again so the difference between the two meanings is more obvious. Both variants have the same genotype, but only the picture on the left side resembles the true look.

A real, typical, but unfortunately not as graphic of an example of co-dominance are the human blood groups.

Blood with the characteristics of AA or A0 react "offensive"  to BB and B0 and vice versa. But the blood type AB reacts with AA, A0, B0 and BB. So the A-part will react with the BB/B0 blood and the B-part will react with the AA/A0 blood.  And that with the same intensity as in the sentence before. Both features are fully expressed and at the same moment.