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The Cultivars (morphs)/Genetics Issues Discussions about genetics issues and/or the various cultivars for cornsnakes commercially available.

Genetics Question
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Old 09-05-2019, 10:12 AM   #1
pitzMike
Genetics Question

Hi all. I had nothing to do last night so I started watching Youtube videos about reptile genetics and some other nerdy stuff. I have watched lessons about genes, locus, allelles and heredity among others.
I understand that when we say a gene is Dominant, it is always in reference to the Wild Type but what about Codominant traits? Does Codominance only works for Allellic Gene Pairs?
I know Ultra/Amel is the only proven codominant genes in corns and that there has yet to be proven Codominant trait in Ball Pythons (since most are incomplete dominant) am I right?
 
Old 09-05-2019, 11:06 AM   #2
Shiari
Not even amel and ultra are codominant actually. They are allelic and incomplete-dominant *with each other*.

Codominant means both genes are expressing at the same time, but not in a combined way. So in the case of, say, a flower that had genes for red and genes for white, the flower would have patches or streaks of red and white, but would not be pink.

Incomplete dominant means the flower would be pink, as both genes are showing a combined expression. Which is why ultra is darker than ultramel which has melanin but amel does not.
 
Old 09-10-2019, 12:09 AM   #3
pitzMike
Quote:
Originally Posted by Shiari View Post

Codominant means both genes are expressing at the same time, but not in a combined way. So in the case of, say, a flower that had genes for red and genes for white, the flower would have patches or streaks of red and white, but would not be pink.
But, isnt BOTH amel and Ultra identifiers shows thru in an Ultramel? Ultra giving it Hypomelanistic Tendencies while Amel affect both the eyes and the overall coloration of the snake?
 
Old 09-10-2019, 10:41 AM   #4
Shiari
No, because the eyes of an ultramel aren't amelanistic. Ultra and amel together make a "pink" flower, not a red and white flower. Ultra is darker than ultramel. Ultramel is darker than amel because some melanin is still present. Amel has no melanin. If ultra and amel were codom the animal would look like a paradox, with amel patches and ultra patches, just like a flower with red streaks and white streaks. But it doesn't show that way. Ultramel shows as an even midpoint between the expression of the two genes.
 
Old 09-10-2019, 08:34 PM   #5
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.
 
Old 09-10-2019, 10:17 PM   #6
Frank Pinello
As clear as mud.......
 
Old 09-10-2019, 10:19 PM   #7
Frank Pinello
......but good job laying it all out.
 
Old 09-10-2019, 11:17 PM   #8
Shiari
A better example of codominance in a cat is actually a tortoiseshell. Orange and black are allelic and on the x chromosome. In order for a female cat to be orange, she must have two orange genes. In order to be black, she must have two black genes. If she has one orange and one black, she shows both colors in patches.
 
Old 09-11-2019, 05:10 PM   #9
paulh
Quote:
Originally Posted by Shiari View Post
A better example of codominance in a cat is actually a tortoiseshell. Orange and black are allelic and on the x chromosome. In order for a female cat to be orange, she must have two orange genes. In order to be black, she must have two black genes. If she has one orange and one black, she shows both colors in patches.
Tortoiseshell
is not an example of codominance. The orange mutant is at the O locus on the X chromosome. The black mutant is at the a locus, which is not on either sex chromosome. The effect of the orange mutant masks the effect of the black mutant in areas where the orange-bearing X chromosome is active. The patchy coat color is an example of X chromosome inactivation, the Lyon hypothesis. See http://cal.vet.upenn.edu/projects/ge...mple/page4.htm
 
Old 09-11-2019, 07:22 PM   #10
Shiari
You learn new things, woot.

So genetically it doesn't work, but the phenotypic appearance still is an example of how codominance *would* word.
 

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