The codominance myth

[CH2O2]

I believe many reptil keepers make a mistake when they say a certain morph is codominant. I'll try to explain why i believe this.

What is a gene?
A gene is a portion of DNA that encodes a protein. In reptiles and many other animals, each gene (with some exception) consists of 2 halves, the mother's half and the father's half.

It is the relation between the 2 halves of a gene that we classify as recessive or dominant.

The morph we call albinism originates from a recessive mutation. The wild type gene responsible for producing melanin (dark pigment) suffered a mutation and thus was created a copy of this gene that does not work (does not produce melanin). A snake that is het for albinism has 1 copy of the functional normal or wild type and 1 copy of the mutation. Since the wild type is functional in the animal, it can produce melanin and we see a normal looking snake. We say the mutation is recessive because, when it is paired with its wild type counterpart is has no effect on the animal. On the other hand, we can say the wild type half of the gene is dominant in regards to its mutant counterpart. The dominant half expresses itself fully in the snake's body (we see a normal animal) while the recessive mutant half does not.

Another example would be the spider mutation. The spider mutation is dominant in regards to its normal or wild type counterpart. A snake het for spider will show the typical spider pattern. There is no visual diference between a het spider and a snake that has 2 copies of the mutation. There is no super morph of spider. We can also say the wild type version of the spider gene is recessive to the spider mutation. In a snake het for spider, the dominant half of the gene expresses itself fully in the snake's body (we see the spider pattern) while the mutant half does not.

What is a codominant gene? A codominant gene is one were both halves express their full potential even when paired together. The human ABO blood system is a good example of codominance. A person with AB blood type produces A type proteins and B type proteins. A is not recessive nor dominant over B and, at the same time, B is not recessive nor dominant over A. Both halves of the gene express themselfs fully, when paired together in the human body.

Now, lets examine the case of the pastel mutation. We can see the pastel effect when the mutation is paired with it's normal or wild type counterpart. A pastel snake is a animal that has 1 copy of the pastel mutation and 1 copy of the normal or wild type.
However, the pastel is not the full expression of the mutation. We only get to see the full expression of the pastel mutation in the super form, when the gene is formed by 2 copies of the pastel mutation. The super pastel is the full expression of the pastel mutation.
In a pastel snake we can say the mutation does not express it's full effect has would be expected if it was codominant.

In a het animal, when the mutation does express itself but not in it's full effect, we cannot call it codominance.

OK, so pastel is not recessive, nor dominant, nor codominant. What is it?

The pastel mutation, like so many other mutations in BP's is incomplete dominant. Somehow snake keeprs forgot this is the correct description for so many beautiful mutations in ball pythons.