paulh

An allele is any of the the versions of one gene. In the corn snake, one set of alleles is made up of the amelanistic, the ultra and the corresponding wild type allele. A second set of alleles is made up of the anerythristic allele and the corresponding wild type allele. The wild type allele in the anerythristic set is not the same as the wild type allele in the amelanistic set. The wild type allele in a given set is the allele most commonly found in the wild population. Only alleles in the same set can form an allele pair.

An allele is always dominant, recessive, or codominant/incomplete dominant to another allele. That second allele may be the wild type (normal) allele or another mutant (abnormal) allele. And the relationship is relative, not absolute. For example, the amelanistic allele is both recessive to the wild type allele and incompletely dominant to the ultra allele. The ultra allele is both recessive to the wild type allele and incompletely dominant to the amelanistic allele. And the wild type allele is dominant to both the ultra and amelanistic alleles. If someone just says that the a given allele is a recessive, a dominant or a codominant, without specifying the second allele, then the comparison is always to the wild type allele. An allele of one gene is never dominant, recessive, or codominant/incomplete dominant to an allele of a different gene.

Two alleles, A and a, produce three allele pairs, AA, Aa, and aa. If allele A is dominant to allele a, allele a is recessive to allele A. The three allele pairs produce two phenotypes (appearances), one for the aa allele pair and one for both the AA allele pair and the Aa allele pair.

Presently, codominance and incomplete dominance have double barrel definitions:
1. In both codominance and incomplete dominance, the three allele pairs produce three phenotypes, one for the AA allele pair, one for the Aa allele pair, and one for the aa allele pair.

2a. In codominance, both alleles produce a functional product at the molecular level.
2b. In incomplete dominance, one allele produces a functional product, and the other allele either produces no product or produces a nonfunctional product at the molecular level.

The problem is the unaided human eye is unable to detect activity at the molecular level. We can infer the answer in some cases.

Red, pink and white (colorless) flowers are the archetypal example for incomplete dominance. Two red alleles produce red (wild type) flowers, and two white alleles produce white (colorless) flowers. A red allele produces a catalyst in the biosynthetic pathway produceing red pigment and a white (colorless, nonfunctional) allele produces no catalyst. Because of the lesser amount of red pigment, the flower looks pink to the unaided human eye. The corn snake's ultra and amelanistic alleles parallel the red and colorless alleles in the flowers.

An example of codominance occurs in cats. There are four alleles, wild type, Burmese, Siamese, and albino. Albino has no pigment, and the others have different amounts of pigment. Wild type is dominant to the rest, and albino is recessive to the rest. A cat with two Burmese alleles is lighter than wild type and darker than a cat with two Siamese alleles. A cat with a Burmese allele paired with a Siamese allele is called a Tonkinese cat and is more or less intermediate in color betwee the Burmese and Siamese cats. As both the Burmese and Siamese alleles produce a catalyst involved in the pigment biosynthetic pathway, the two alleles are codominant to each other.

The ball python has three alleles, wild type, lesser, and mojave. A snake with two lesser alleles has no pigment. Presumably, the lesser allele does not produce a functional product. A snake with a lesser allele paired with the wild type allele is not white but is much lighter in color than a snake with two wild type alleles. This parallels the red, white and pink flowers, so we can say the lesser allele is incompletely dominant to the wild type allele. On the other hand, a snake with two mojave alleles is pale grey, so the mojave allele produces a functional product, even if that product is not as effective as the wild type allele's product. A snake with a mojave gene paired with a wild type allele is lighter in color than a wild type ball python and darker than a snake with two mojave alleles. This parallels the condition in the Burmese, Tonkinese and Siamese cats, so the mojave allele is codominant to the wild type gene.

QED, both codominance and incomplete dominance occur in ball pythons.

Now let's look at the wild type allele and the pastel allele in ball pythons. A super pastel ball python has two pastel alleles. Such a ball python is lighter than a wild type ball python but not white. A pastel ball python has a pastel allele paired with a wild type allele. It is lighter in color than a wild type snake and darker than a super pastel. That satisfies the requirement that each allele pair has its own phenotype. But does that pastel allele produce a functional or nonfunctional product? Unless one allele produces a colorless phenotype, We don't know the answer. We don't know the answer to the same question about most of the mutants we usually call codominants. Some people prefer "incomplete dominant" or "partial dominant" instead of "codominant". YMMV.