Reptile & Amphibian Forums

As one whom teaches genetics, why do we say the inheritance of hypomelanism is codominant? Codominant inheritance results in the equal expression of both alleles at the molecular level which in turn is usually expressed phenotypically, such as in blood types (e.g. AB, Ai, Bi). From the literature I have read, hypomelanism in Boa constrictors (to date) is referred to as incomplete dominance. Incomplete dominance is not as extreme as codominance, in that the phenotypic expression lay somewhere between the wild type (normal) and the mutant. I believe for our hobby/industry(breeding) to receive respect and recognition, we should clear up the language we use to describe what we do (if you are a morpher). Minimally, hypos should be described as being the result of incomplete dominance due to the array of hypos out there (i.e. evidenced by the varying degree of hypomelanism in said animals). At most, we should simply say hypomelanism is a dominant trait.

Here are some problems people will have with hypomelanism being a dominant trait: in dominance-based inheritance, the heterozygote is normally indistinguishable from the wild type and would have to go through a test cross to identify its genetics. Furthermore, how do we definitively distinguish between heterozygote and homozygote mutants (aside from the cleaner and more aberrant hypos, which does not always work out).

The hypo thing is a bit of an anomaly, but until it is fully understood (i.e. the molecular basis of the mutation) I feel it is better to say the mutation is incompletely dominant or simply dominant (with revision).
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Ron Michelotti

Class Reptilia
www.classreptilia.com

I do not believe that the hypo gene is co-dominate I have only used the term to avoid confusion. It is a wierd morph in the fact that it basically acts like co-dominate trait but it also affects the appearance part of the litter that are not hypos. I would find the results of a breeding between two hypo siblings interesting. But I do not believe that dominate is the correct term for it either, unless it is a proven dominate hypo. I believe incomplete dominate is the closest to correct. Hope I confused someone else as much as I did myself.
Chris

In homozygous form the transmissive result has been proven to be "complete" for roughly 10 years,or,so now?...I just don't see the evidence for the "incomplete dominance" theory I guess?...in your post you said: (quote) "Here are some problems people will have with hypomelanism being a dominant trait: in dominance-based inheritance, the heterozygote is normally indistinguishable from the wild type and would have to go through a test cross to identify its genetics." (end quote) The heterozygotes of a recessive trait are visually indistinguishable,however,the hets of a dominant trait express the trait,and,are easily distinguished from the non-gene carriers,that's the single foremost criteria distinguishing it from "recessive transmission".so,I don't understand that portion of your post? I freely admit that I have no formal education in genetics,or,biochemistry,so it's possible that I just don't understand from a terminology standpoint? I believe,initially,people needed a term to describe the results seen from breeding the heterozygous animals to non-gene carriers,this resulted in 50% het/50% without trait.("normal" ) this led to the (informal?)classification of the trait as "incomplete dominant",or,(later)"co-dominant",once the trait proved that the production of a completely dominant,"homozygous" form could be achieved,everyone continued to cling to the previous notion that it was "co-dominant"...often the evidence evolves faster than our ability to label,and,comprehend it.

in what I said (and you quoted). A heterozygote for a recessive mutation is indistinguishable from the wild type (e.g. het albinos), but heterozygote boas for hypomelanism are hypomelanistic and easily distinguished from the wild type.

That is what I get for trying to hurry a post between classes

The point I was trying to make is the dominant phenotype (in this case hypomelanism) is not black and white, like albinism (disregarding any underlying colors which make some albinos better looking than others) or red or white flowers in a previous post. I even had a hypo litter where there were a couple of animals on the fence- were they hypos or not? We are looking at a range of expression of this gene, as evidenced by the different looks of hypos. There is a strange breakdown in the melanin producing pathway, which yields different looking hypos. In addition, hypos still produce a certain amount of melanin (including homozygous dominant animals). To say it is purely a dominant trait does not seem right. There is an incomplete masking of hypomelanism over the wild type- and remember incomplete dominance is still "dominance," but where one allele is more prevalent than the other.

Sorry for the confusion.
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Ron Michelotti

Class Reptilia
www.classreptilia.com

Ok,I understand what you're saying...We've also seen numerous individual albino animals that could be described accurately as "intermediate" (also "paradoxical" ) so these laws are not without exceptions to what we know to be the "rules",even when they are usually "black & white"...regardless of these enigmatic exceptions to the rules,(I say enigmatic,because they are anomolies within an anomoly.)we still agree that albinism is "simple recessive",had further breeding of Salmon/Orange-tail animals never resulted in a proven dominant form of the trait,I could certainly see classifying them as "co-dominant",or,"incomplete dominant",because that is what the initial results showed us,however,once we were able to prove homozygous expression,my contention is that the trait itself became reliably,and,completely "dominant",the only thing "incomplete",or,"co-dominant" about it is found in the heterozygotes...when you breed a homozygous Salmon to a non-gene carrier all of the offspring are salmon,het for homozygous "hypo",regardless of individual variance,or,trait penetrance within the litter...if you breed the less impressive expression animals to "normals" you will get 50% "hypos"/50% normals,proving them to be hets. I'm quite sure there could be an occaisional exception to the rule,just like there have been in every other genetic mutation,but,I think we've seen quite a bit of reliable predictability with it over the years...I just think it's time we allow our minds to evolve with the proof?

> (snip) once we were able to prove homozygous expression,my contention is that the trait itself (hypo) became reliably, and,completely "dominant", the only thing "incomplete",or,"co-dominant" about it is found in the heterozygotes... (snip)

You have a misconception of the definition of dominant vs. codominant.

A mutant gene is called a codominant if the heterozygous individual (with one normal and one mutant gene) can be reliably distinguished from the homozygous individual (with a pair of mutant genes)

A mutant gene is called a dominant if the heterozygous individual looks like (or cannot be reliably distinguished from) the homozygous mutant individual.

So if you can usually tell an individual with one mutant gene and one normal gene from an individual with two identical mutant genes, then the gene is a codominant mutant. The gene is a codominant mutant gene whether a particular individual has one mutant gene or two mutant genes. And a dominant mutant gene is a dominant mutant whether a given individual is heterozygous or homozygous for that gene.

All I know about salmon comes from what I've read. So I'm not positive whether the salmon mutant should be called a dominant or a codominant mutant. IMHO, if you can put known salmons and supersalmons in a bin and have a reasonably experienced person separate them with ~95% accuracy, then the salmon gene is best called a codominant mutant gene. If accuracy is more like 50% or lower, then I'd say that salmon should be called a dominant mutant gene with variable expressivity.

And Ron, "codominant" and "incomplete dominant" have different meanings on the molecular level. At the coarse level we are working with here, both mean that the genotype can be identified from the phenotype. Other synonyms include semidominant, partial dominant, less than dominant, etc. Consistency is desirable, but which synonym you use isn't a life-threatening matter. I prefer "codominant" because it is the shortest of the lot.

Paul Hollander

(I was hoping you'd show up! )To be honest,I think based on your information,the Salmon/Orange-tail mutation would best fit the definition of dominant,for the exact reasons you explained.I feel certain that someone with a level of experience breeding,and,assessing several hundreds of individuals over the course of time,could probably achieve the percentage level of 90/95%(like Rich,for example.) in accurately determining homozygous from heterozygous expression,maybe even higher? I also think that if I were able to view an entire litter containing both hets,and,"supers",I could probably score fairly high as well?(probably not 90% though?) however,the simple definition,for the sake of discussion,would be that with a "dominant" mutation,both heterozygous,and,homozygous forms look the same,this is why we say "possible super" in presenting an animal for sale from parents that also express the trait.(this may have been less true in the begining,but,at this stage of their production,there are an increasing number of "F1" hets,that lack melanin below the vent,and,exhibit extreme pattern reduction,(two of the primary visual ques used to differentiate between the two.)making it difficult for the average person to tell them apart with any real degree of certainty.)Something like what is currently being seen with the Jungle mutation,comes much closer to the simple definition of "co-dominant",(at least the way we are using it?)in that,the heterozygous form looks different from WT,but het,and,homozygous are dissimilar in appearence? without delving into the molecular,wouldn't the simplest,and,most straightforward definitions serve our interests best?(I'm just asking,*lol* I'm not trying to change the world here! )In your view,what would be a clear cut example of a "dominant" trait expressed in reptiles?

I also believe that the simplest and most straightforward definitions serve our interests best. The genetics prof that I worked for some years back did too. He didn't split hairs over codominant vs incomplete dominant vs partial dominant, etc. He just lumped them all together. A quick and dirty solution, but that's all we have data for in most cases.

Normal vs tiger (heterozygous tiger) vs super tiger (homozygous tiger) in the reticulated python is one of the best examples of a codominant mutant gene that I know of in snakes.

I don't know of a really good example of a dominant mutant gene in snakes. I could do much better with pigeons or mice. It is possible that spider in the ball python will turn out to be a good dominant, but I haven't heard whether anybody has bred a homozygous spider for sure. But that's the way things are when almost all our snakes are less than 20 generations out of the wild.

Paul Hollander

You taking the time to write back,I also take your experience in genetics to heart,and,am glad to have your input on the subject.I've always considered variable expressivety to be a natural occurence within almost every mutation,albiet much less a factor in something like T neg. amelanism,for example...I can certainly understand combining several very similar terms in favor of a single,all-inclusive term,as you mentioned...it's just that when looking at the "Salmon/Orange-tail" situation,I fail to see it fitting the simplest defining criteria for catagorical inclusion into the grouping...weighing that with its overall consistency across the board,it just doesn't add up...I've seen numerous homozygotes that appeared less intensely expressive than many hets,so the fact that it can present any doubt at all in discerning between the two represents reasonable reconsideration to my way of thinking? The way we are using the terminology within the hobby,we are describing the "intermediate" (hetero.) form as "co-dominant",not because of the definitions for classification,but,because of the way it is transmissively passed,we then refer to the homozygous form as "dominant",again,not because of the definitive classification,but,because of the transmissive nature of that form of the trait...it's like saying albinism itself is recessive,but the "intermediate" form is "co-dominant" because it results in a 50/50 transmissive split??? we should simply say,based on definition,that "hypos" (what we are calling "hypo" in boas.)are dominant!?(if the scientific community needs to add: "with revision" to be more accurate,I think that's acceptable?)we're not using "co-dominant" for any of the right reasons,and,the trait itself is not cooperating with that classification.(*lol*)

>snip) I've seen numerous homozygotes that appeared less intensely expressive than many hets,so the fact that it can present any doubt at all in discerning between the two represents reasonable reconsideration to my way of thinking?

When Rich Ihle's paper appeared in the Journal of Heredity, he called salmon a codominant mutant. Based on his observations, it seemed to fit a standard definition of codominant -- that the genotype can be told from the phenotype. You and others say that in too many cases the heterozygous and homozygous salmons cannot be distinguished. This is good reason for changing salmon's classification from "codominant" to "dominant".

Books and journals have margins so that people can pencil in corrections.

> The way we are using the terminology within the hobby,we are describing the "intermediate" (hetero.) form as "co-dominant",not because of the definitions for classification,but,because of the way it is transmissively passed,we then refer to the homozygous form as "dominant",again,not because of the definitive classification,but,because of the transmissive nature of that form of the trait...it's like saying albinism itself is recessive,but the "intermediate" form is "co-dominant" because it results in a 50/50 transmissive split???

Right on. The standard term for the condition resulting in a 50/50 tranmissive split is "heterozygous". A het albino has a 50% transmission of the albino mutant gene and 50% transmission of the normal gene. Het salmon has a 50% transmission of the salmon mutant gene and 50% transmission of the normal gene. That is why I strongly oppose the statement that there is no such thing as het salmon.

Thank you for this example. I will add it to my collection of arguments for the existance of het salmon and other dominant/codominant mutants.

> we should simply say,based on definition,that "hypos" (what we are calling "hypo" in boas.)are dominant!?(if the scientific community needs to add: "with revision" to be more accurate,I think that's acceptable?)we're not using "co-dominant" for any of the right reasons,and,the trait itself is not cooperating with that classification.(*lol*)

Yes. As salmon's expression is not cooperating with the definition of "codominant", we need to see whether it will cooperate better with the definition of a different term. Salmon is definitely not a recessive mutant gene, and the only other choice is "dominant".

Unfortunately, nature is sloppy. Salmon seems to be one of the mutants that falls into the gray area between man-made definitions. We have to decide which definition is a better fit, rather than which definition is a perfect fit.

Paul Hollander