>So in both co-dominant and incomplete-dominant both traits are
expressed but in the co-dominant form the more dominant alle
(motley in the case of motley boas) overrides the other 'normal'
alle and in incomplete-dominant the two alle/traits are blended together. Do I have it yet?

You don't have it quite yet.

First of all, traits are what you see. For example, motley is the name of a trait (a pattern) and the name of a mutant gene. Genes are in the cell. Genes produce various chemicals that interact with the biochemical machinery in each cell and ultimately produce the traits that we see. Genes are passed to the sperm and the egg, but melanin is not passed to the sperm and egg. So the gene that produces the motley pattern is inherited, but the actual motley pattern of markings (the trait) is not inherited.

A DOMINANT allele masks the presence of a RECESSIVE allele. In a boa with an albino mutant allele paired with a normal allele, the normal allele masks the presence of the albino mutant gene. The boa looks normal. That makes the normal allele dominant to the albino allele and the albino allele recessive to the normal allele. In a boa with a salmon mutant allele paired with a normal allele, the salmon mutant allele masks the presence of the normal allele. The boa looks salmon. That makes the salmon allele dominant to the normal allele and the normal allele recessive to the salmon allele.

With dominant and recessive alleles, there are TWO traits. There is one trait for the animal that is homozygous for the recessive allele (a pair of identical recessive alleles). And there is one trait shared by the animal that is homozygous for the dominant allele (a pair of identical dominant alleles) and the heterozygous animal (with a dominant allele paired with a recessive allele).

With both CODOMINANT and INCOMPLETE DOMINANT alleles, there are THREE traits. One trait for one homozygous animal (with a pair of identical copies of one allele). One trait for the other homozygous animal (with a pair of identical copies of the other allele). And the third trait for the heterozygous animal (with a pair of genes made up of one copy of each of the two alleles). Lumping the two categories together is actually good enough for breeders' and sellers' purposes. I follow my old genetics prof in calling that lumped together category "codominant" because it is shorter than "incomplete dominant" and much shorter than "codominant/incomplete dominant".

By convention, we call the normal allele the standard of comparison with the mutant allele assigned the dominant or recessive or codominant position. If there are three or more alleles, each pair of alleles is compared.

If you really want to get into the morass of verbiage required to explain the difference between "codominance" and "incomplete dominance", then continue. You might prefer to stop here, though.

Biochemistry and molecular genetics mostly take place inside a single cell. And all that gets really complicated really fast. Molecular genetics and the phenogenetics involved in breeding boa morphs are two different levels of organization.

On the molecular genetics level, genes do their own thing. The albino mutant gene does its own thing and the albino mutant's normal allele does its own thing whether there are two albino mutant genes, two normal alleles, or an albino mutant gene paired with a normal allele. The same is true for all genes, whether mutant or normal or whether dominant, recessive, codominant or incomplete dominant. In other words, if there are two different genes in a gene pair, each one produces its own product inside the cell. The two products do not form an intermediate or blended product. It is the mixture of those two products in each cell that gives rise to what we see in the whole animal.

Lets say that we have an individual snake that has a codominant mutant gene paired with a normal allele. If we could look inside each cell, we would see a mixture of two different gene products. One product would be made by the mutant gene, and the other product would be made by the normal allele. If we looked at the whole animal, we would probably see a blended or intermediate effect in comparison to a snake that has two mutant genes and a snake that has two normal alleles.

Lets say that we have an individual snake that has an incomplete dominant mutant gene paired with a normal allele. If we could look inside each cell, we would see a mixture of two different gene products. One product would be made by the mutant gene, and the other product would be made by the normal allele. If we looked at the whole animal, we would probably see a blended or intermediate effect in comparison to a snake that has two mutant genes and a snake that has two normal alleles.

If you read the last two paragraphs closely, you will notice that the only difference is that one says "a codominant mutant gene" and the other says "an incomplete dominant mutant gene".

By now you are probably wondering whether there really is any difference.

Here is the difference:

In codominance, both alleles make products that do something in the biochemistry. It is the mixture of two functional products that causes the differences between the heterozygote and the two homozygotes.

In incomplete dominance, only one of the two alleles makes a product that does something in the biochemistry. The other allele makes a product that does nothing in the biochemistry. And it is the different amount of functional product in the heterozygote that causes the differences between the heterozygote and the two homozygotes.

>Are there any incomplete-dominant traits out there?

There are no incomplete dominant traits, no codominant traits, no recessive traits, and no dominant traits. There are dominant, codominant, incomplete dominant, and recessive mutant GENES. The genes cause the traits that we see. That is a difference between "trait" and "gene".

Some mutant genes are incomplete dominants, and some mutant genes are codominants. That is, in the strict sense of the functionality of the gene products as given above. But I cannot give any examples in the boa constrictor. Because I do not know whether or not a given mutant gene's product is functional. We can tell whether a heterozygote can be routinely distinguished from the two homozygotes, but that simply returns to lumping incomplete dominants and codominants into one category rather than separating them.

Hope this helps.

Paul Hollander

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