The codominance myth

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.

I think that for the most part most people realize that the term codom is not exactly correct. Its more or less a schema that allows them to easily organize and describe the way certain genes act along with their visual traits without getting into the technicalities of genetics. Alot of people use the term incomplete dominant as opposed to co dominant.

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Originally Posted by CH2O2

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.

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Well put...though a lot of people do actually know that a "co-dom" in the ball world is actually an incompete dominant gene.

My opinion is that since by far, most keepers/breeders have a very limited scope on genetics, myself included...that incorrect or not, things tend to stick once said over and over. Look at morphs that express hypomelanism as one of their defining traits, yet only ghosts are referred to as hypos.

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Originally Posted by Domepiece

I think that for the most part most people realize that the term codom is not exactly correct. Its more or less a schema that allows them to easily organize and describe the way certain genes act along with their visual traits without getting into the technicalities of genetics.

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I'm not sure most people realize the term is incorrect. Anyone can use the correct term even if they don't want to go into the technicalities of genetics. I'm convinced many reptile keepers don't know there is a thing called "incomplete dominance", even among those who dare to go into the technicalities.

I believe the problem existes because (as far as i know) BP's don't have any codominant morphs. Snake keepers never had to make the distinction between codominant and incomplete dominant. When someone first used the term codominant it got stuck in people's minds.

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Originally Posted by RobNJ

things tend to stick once said over and over. Look at morphs that express hypomelanism as one of their defining traits, yet only ghosts are referred to as hypos.

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I agree. As the genetics junky that i am, i believe forums like this are the perfect oportunity to shine a light on these issues.

This has been brought up before.

ABO blood typing is multiple allele series similar to say the BEL or YB complexes, meaning there is more than one allele at work, and only 2 can be present at a given time. So, if you have A & B you are then AB, but if you get say A & O you are A (but can pass a O) and if you are A & A you are A (and can only pass A). And, as you stated, this is considered co-dominance.

The argument becomes does the "super" of a BP color mutation form act as a blend (incomplete dominance) of the genes OR as a expression of both equally (co-dominance). I am not sure if that is really clear in this case. And, with multi allele series and different apparent strengths of the genes, it complicates the issue even further.

Either way, In the case of the BP the whether or not is it called CoDom or Incomplete Dom really is moot unless there are some other factors that are found to come in to play. Meaning either provides a useful description of the phenomenon.

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Originally Posted by CH2O2

I'm not sure most people realize the term is incorrect. Anyone can use the correct term even if they don't want to go into the technicalities of genetics. I'm convinced many reptile keepers don't know there is a thing called "incomplete dominance", even among those who dare to go into the technicalities.

I believe the problem existes because (as far as i know) BP's don't have any codominant morphs. Snake keepers never had to make the distinction between codominant and incomplete dominant. When someone first used the term codominant it got stuck in people's minds.

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I'll rephrase that there are quite a few at least on this forum that understand the difference between the two. Also I agree that old habits are hard to break and people just use co dom out of habit.

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Originally Posted by jinx667

This has been brought up before.

ABO blood typing is multiple allele series similar to say the BEL or YB complexes, meaning there is more than one allele at work, and only 2 can be present at a given time. So, if you have A & B you are then AB, but if you get say A & O you are A (but can pass a O) and if you are A & A you are A (and can only pass A). And, as you stated, this is considered co-dominance.

The argument becomes does the "super" of a BP color mutation form act as a blend (incomplete dominance) of the genes OR as a expression of both equally (co-dominance). I am not sure if that is really clear in this case. And, with multi allele series and different apparent strengths of the genes, it complicates the issue even further.

Either way, In the case of the BP the whether or not is it called CoDom or Incomplete Dom really is moot unless there are some other factors that are found to come in to play. Meaning either provides a useful description of the phenomenon.

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I think it is clear for the color mutations. If it was a codominant mutation, Mojave's, Butter's, etc should have BEL and wild type coloring, not what we see. Pastel's should have patches of super pastel coloring and patches of wild type coloring. Picture something similar to the Pied Albino Paradox pattern. The expression of the gene is being reduced in het form so it must be incomplete dominance.

I agree though that it doesn't really matter. Codominance is being used as an industry/trade term to describe morphs, not as a scientific one. I suspect many people do realize it's really incomplete dominance but choose to say codominant since this is the commonly accepted term.

Seriouslly whats the point? However you want to say it those of us who breed know what snakes to put together to produce what we are going for. This is just gonna turn into one of those 8 page threads about tomato timoto.

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Originally Posted by jinx667

ABO blood typing is multiple allele series similar to say the BEL or YB complexes, meaning there is more than one allele at work, and only 2 can be present at a given time. So, if you have A & B you are then AB, but if you get say A & O you are A (but can pass a O) and if you are A & A you are A (and can only pass A). And, as you stated, this is considered co-dominance.

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I'm not sure you are making this argument but the fact that the ABO system is a multiple allele series does not change anything. Again, i'm not sure you are defending this. Even for a multiple allele series, a given individual will still have 2 and only 2 of the possible alleles in the series. It is the relation between those 2 alleles that we consider to be dominant or recessive.

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Originally Posted by jinx667

The argument becomes does the "super" of a BP color mutation form act as a blend (incomplete dominance) of the genes OR as a expression of both equally (co-dominance).

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I agree.

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Originally Posted by jinx667

I am not sure if that is really clear in this case.

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To make it clear we must look into the definition of codominance and incomplete dominance.

Codominance - both alleles in the gene express fully in the phenotype.
Incomplete dominance - the phenotype is a intermediate form of the effect each allele has in the homozigous form.

If you agree with the definitions, you should come to the conclusion that the pastel phenotype is an intermediate phenotype between the normal and the super pastel. Thus, it should be classified has incomplete dominante.

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Originally Posted by jinx667

And, with multi allele series and different apparent strengths of the genes, it complicates the issue even further.

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Again, we must consider the relation of the alleles in pairs. AB = codominance,
AO and BO are domimant/recessive where O is recessive to both A and B.

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Originally Posted by jinx667

Either way, In the case of the BP the whether or not is it called CoDom or Incomplete Dom really is moot unless there are some other factors that are found to come in to play.

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I do not agree. By definition, one gene is either codominant or incomplete dominant. No other external factors, apart from the definition, will ever change that.

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Originally Posted by jinx667

Meaning either provides a useful description of the phenomenon.

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Again i do not agree. Codominance and incomplete dominance are two very distinct and different phenomena. What happens is that people became accustomed to describing one phenomenon using the term that describes another phenomenon.