Reptile & Amphibian Forums

This is in response to a post below
I ran out of space, so there will be a few follow up posts.

**As said before, I am not a geneticists but I have taken several genetics, population genetics, and evolutionary biology courses.

Genetics 101 (as I understand it)

Genes are found on chromosomes and their location on a chromosome is called the locus (loci = plural). There are often several different mutations of a single gene, and these are referred to as alleles. The genotype is the combination of these alleles for a given character (melanin production for instance). The phenotype is the observable characteristic or trait of an organism (e.g. reduced melanin/amelanism/albino), which is often dependant on the genotype.

Nomenclature is a construct of man. It is way for humans to categorize things so they are easier to understand. In genetics, I am not aware of a single term specifically created to represent a doubly homozygous recessive genotype. The phenotype however can be called whatever we want, for example "Snow" or "Ghost". Realistically the genotype is what is important, not the phenotype.

Now, it is worth noting that NOT all phenotypic characters come from single-gene interacts like the examples follow. Human eye color for instance is control by multiple genes, also known as multiple gene interactions (check out google for more info.). Line-bred traits are traits that are likely control by multiple gene interacts, that for now, are too complicated for breeders to predict.

As far as resources go. Most of my knowledge comes from the classes I have attended and I cannot cite them directly.

A good entry-level (college) genetics book I have is:

Brooker, Robert. 2008. Genetics: Analysis and Principles. 3rd Edition. McGraw-Hill Science. p864. (You could probably go with an older addition to save money).

You could also check out:

Genetics for Dummies
Evolution for Dummies

Both can be purchased for less than $20 ea.

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Christopher E. Smith
Contact
Captive Bred Herps
Wildlife Research & Consulting Services, LLC

Example 1
Axanthism in Heterodon nasicus for instance has been discover from at least 2 locales (i.e. BHB line and Russo line). When crossed, we would expect normal offspring in the F1 generation if the gene(s) controlling axanthism are on 2 separate loci. The offspring in this scenario would be doubly heterozygous (heterozygous at both loci). However, it has been proven that crossing the two lines will produce axanthic individuals, thus we can deduce that it is the same gene at the same location on the chromosome (locus) controlling the mutation.

Example 2
There are several lines of amelanism in H. nasicus and we know that at least some of these ‘lines’ are not compatible. For instance, if you breed a T-plus albino (traditional hypo), to a T-minus albino (traditional albino), you will get ALL normal looking babies (these babies would be doubly heterozygous). This is because the genes that control the reduction in melanin are on two separate loci.

Troy- Feel free to contact me if you have more questions or would just like to discuss genetics.

-Chris
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Christopher E. Smith
Contact
Captive Bred Herps
Wildlife Research & Consulting Services, LLC

np
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Christopher E. Smith
Contact
Captive Bred Herps
Wildlife Research & Consulting Services, LLC

I followed the thread below, but thought it prudent to post my reply up here. The combination of two homozygous recessive traits in a single individual is just that: the combination of two (or however many the case may be) in a single individual. It is a phenomenon much different than co-dominance or incomplete dominance, since the expression of one trait does not have an effect on the expression of the other. For example, the homozygous expression of amelanism (we'll call it aa - the designation is really arbitrary) does not have an effect on the homozygous expression of axanthism (we'll call it xx). And animal displaying both traits would be aa AND xx, with the result being a "snow".

Here's a link to Vinny Lynch's website, where he has a pretty killer explanation of squamate genetics... and rightly so, since he's a geneticist... Anyway, it explains simple recessive traits, co-dominance, incomplete dominance, and multigenic traits in fairly plain language.

Natural Selection Reptiles

-Cole

Nomenclature is a construct of man. It is way for humans to categorize things so they are easier to understand. In genetics, I am not aware of a single term specifically created to represent a doubly homozygous recessive genotype.

While I too am currently unaware of a term that defines the unique phenotype of 2 or more combined Recessive traits resulting in a homozygous animal, the two closest terms, but yet differently defined would be dihybrid and polygenic (otherwise known as linebred traits).

Dihybrids are often explained using the model of a Dominant trait mixed with a Recessive non-related type of trait. When I say non-related, I mean that the traits are different in terms of physical characteristics such as one trait being blond hair while the other is blue eyes. Obviously, would not apply because what we are talking about with Snows are all color related traits. I would ponder though whether it would apply to something like an Albino Anaconda/Superconda as the Albino is one type of trait (color) while the Anaconda/Superconda is another type (pattern reduction), I believe it would.

Polygenic traits are something entirely far more complex being traits that are expressed from multiple genes residing at different loci on different chromosomes. Polygenic traits are far more complex than the simple Recessive traits that we are discussing such as a Snow but Polygenic traits are generally attributed to phenotypes such as the red coloration in an Extreme Red Albino and all of the variations that these consist of.

The reason why I mention that a Double Recessive Homozygous animal, like a Snow, is kind of like an Incomplete Dominance type of condition is because of the blending condition that appears to be happening. For those that didn't read my previous indepth post on Dodominance and Incomplete Dominance, please read this: Codominance & Incomplete Dominance. Also be sure to read the references I cite at the bottom of my post if you in fact question what I said in that post.

The phenotype however can be called whatever we want, for example "Snow" or "Ghost". Realistically the genotype is what is important, not the phenotype.

While I do agree with you in general, the one time that the genotype isn't necessarily important is when epistasis occurs. Of course this typically only happens with animals such as Leucistics, so it will be interesting when the Leucistic Hognose emerge how, or if, they can be implemented with other morphs. And while you state "Realistically the genotype is what is important, not the phenotype.", it's the phenotype that commands the price, which is why hets are usually 50-75% cheaper than the phenotypic animals.

Now, it is worth noting that NOT all phenotypic characters come from single-gene interacts like the examples follow. Human eye color for instance is control by multiple genes, also known as multiple gene interactions (check out google for more info.). Line-bred traits are traits that are likely control by multiple gene interacts, that for now, are too complicated for breeders to predict.

Agreed, these are generally termed as Polygenic traits, meaning many genes.

A good entry-level (college) genetics book I have is:
Brooker, Robert. 2008. Genetics: Analysis and Principles. 3rd Edition. McGraw-Hill Science. p864..

Thanks for the recommendation. I was actually looking at purchasing that book a few weeks ago to better supplement these two books I've been reading from to gain my knowledge:
Biology: Concepts & Connections
Genetics For Herpers
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Troy Rexroth
Rextiles

So while I agree with the definitions of incomplete and co-dominance that you posted earlier (and have been harping on their use in herps for a while), they only refer to the expression of alleles at a single locus not the expression of alleles at different loci (genes). So, while we would say that the anaconda allele is incompletely dominant with respect to the wild-type pattern and itself, we would not say that its co-dominant to albino because you can have albino anacondas. They are not co-dominant since different genes (loci) are responsible are each trait. Similarly, my brown hair is not co-dominant with my ability to taste bitter things because different genes are involved.

The best example of co-dominance I can think of are roan coat color in horses and cattle and blood types. Roan coats look pink or spotted, because there is dark hair and light hair in the same animal. Similarly you can have a blood type AB because you carry the gene for both type A and type B and both proteins are expressed on your blood cells.

Vinny
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“There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that whilst this planet has gone on cycling according to the fixed laws of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” -C. Darwin, 1859

Natural Selection Reptiles

Hi Vinny, thanks for joining in!

So while I agree with the definitions of incomplete and co-dominance that you posted earlier (and have been harping on their use in herps for a while), they only refer to the expression of alleles at a single locus not the expression of alleles at different loci (genes).

That makes sense to me. Now I just need to try and comprehend it in terms of trying to determine what definition(s) a Double Recessive Homozygous animal would be, if one even currently exists. I don't see why such a term wouldn't exist though considering that terms are basically defined for every other kind of trait and that something like this is defintely nothing new or out of the ordinary although a bit more rare. So surely there must be a term or phrase that properly defines this blending phase for multiple Recessive trait Homo animals along the lines of how an Incomplete Dominant trait acts for Dominant traits.

So, while we would say that the anaconda allele is incompletely dominant with respect to the wild-type pattern and itself, we would not say that its co-dominant to albino because you can have albino anacondas. They are not co-dominant since different genes (loci) are responsible are each trait. Similarly, my brown hair is not co-dominant with my ability to taste bitter things because different genes are involved.

I agree with this as well. I hope I didn't say anything that could be misconstrued into thinking that I thought just the opposite, that mixing a color trait with a pattern trait should be considered a Codominant or Incomplete Dominant type of phenotype. I only referred to the blending mix of 2 Recessive color traits as acting like an Incomplete Dominant type. I believe in one of the posts I made yesterday that I stated along the lines that an Albino (color trait) Anaconda (pattern trait) would more than likely classify it as a Dihybrid type.

I'll just reiterate once again that I misspoke about saying that a Snow was an Incomplete Dominant type of trait. I should have said that it was merely acting like one and left it open to discussion.

Thanks again for your input Vinny. I believe that all of us are learning valuable insights and gaining a deeper understanding from each other in regards to comprehending this very complex topic.

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Troy Rexroth
Rextiles

Troy,
You bring up some good questions that are rearely asked...

Another point that has suprizingly not been brought up is the fact that when two different recessives are bred together, the resulting offspring are normal in color... So you see no results in the first generation... With incomplete dominance you see the results in the first generation...

When an animal is doulble honozygous the reason you are able to see both traits is because the two recessives ride on different loci...
With incomplete dominance, the blending happens on the same loci...

At least that how I understand it...

NP

Troy,
You bring up some good questions that are rearely asked...

Thanks Gregg!

I truly hope that nobody thinks that I'm trying to challenge the establishment (meaning all of us reptile breeders) or worse, trying to redefine the standards of genetics. If anything, I'm just trying to get a clear understanding of something that is either overly complex or so watered down that either way forces most of us into making interpretations based on our own presumptions. I will say that some of these topics do challenge the tenets long held by some in the herpetocultural world where a change of ideas is not so easily or readily accepted. I can only hope that through respectful debate and diligent research can we come to a more informed and correct consensus about such complex topics. While the internet is an invaluable source of information, it's also a terrible repository of misinformation, especially in some of the more popular forums that I've come across and/or belong to.

Another point that has suprizingly not been brought up is the fact that when two different recessives are bred together, the resulting offspring are normal in color... So you see no results in the first generation... With incomplete dominance you see the results in the first generation...

Correct, Incomplete Dominance is specifically defined by the appearance of the F1 generation unlike the Double Recessive Homo (DHR) that will only result from an F2 generation (or later based on the odds of producing such a mutation).

When an animal is doulble honozygous the reason you are able to see both traits is because the two recessives ride on different loci...
With incomplete dominance, the blending happens on the same loci...

Yes, I believe that is what I failed to recognize when I made the statement that the DHR's are showing Incomplete Dominance instead of asserting that they are merely mimicking the blending aspect of Incomplete Dominance.

While I still don't fully agree that a Snow is literally showing both Albino and Axanthic colors at once and yet somehow appear to blend and come across as a Snow colored individual, to me, I presume that the blending is happening in much the same way as it does in Incomplete Dominance traits despite the gene loci and recessiveness of the traits, I just can't understand how it would happen otherwise. However, I wouldn't argue that the genotype could be interpreted in that way considering that both the Albino genes and Axanthic genes are paired in a way that would be phenotypic of either of those individualistic mutative colors. So, from that perspective I can understand how some do come to the conclusion that both colors are being shown at the same time which is resulting in the blending of colors instead of the blending taking place on a more genetic level as I presume.

At least that how I understand it...

Hey, everybody's input is very welcome. I know I am learning things, either directly or indirectly, from everything that is being said in these threads by all of you choosing to participate on this topic. I find it mentally stimulating and very worth the time I've spent researching and corresponding with all of you; I hope you all feel the same. Thanks again for sharing your time and knowledge!
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Troy Rexroth
Rextiles

While I still don't fully agree that a Snow is literally showing both Albino and Axanthic colors at once and yet somehow appear to blend and come across as a Snow colored individual, to me, I presume that the blending is happening in much the same way as it does in Incomplete Dominance traits despite the gene loci and recessiveness of the traits, I just can't understand how it would happen otherwise.

Ah, I think I see what you are getting at now. A snow, for example, is actually not "showing" anything. Neither are albino nor axanthic for that matter. Its an absence of both red and black, since these pigments are missing what we are observing can not be a blending of something since there is no red or black pigment to blend.

This can happen because red and black pigments are chemically very different and are produced by different pathways in the cell. Different cells are responsible for making the different pigments.

Its kind of like a pixelated picture, seen from very close all you can see are individual dots of color. But seen from very far away they appear to blend to produce an image and you can no longer see the individual dots. They are still there as individual dots of color, but our eyes do not have the resolution to see them. What we are seeing from far away is actually an optical illusion. Were we to have eyes and a brain that could process really high resolution images we might actually see a snow differently, perhaps instead of the optical illusion of blending we would see dots (cells) of missing red and dots (cells) of missing black on a background of dots of some other color.

I hope I did not go too far over the edge with that explanation
Vinny
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“There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that whilst this planet has gone on cycling according to the fixed laws of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” -C. Darwin, 1859

Natural Selection Reptiles

With the axanthics/anerys, are the hogs actually either of those???
By genetic definition, Axanthics should produce no yellow coloration... With anerys, by definition, there is no red coloration produced...

From what we see in the hogs, they do not produce either coloration which is obvious when looking at the belly where there should be orange patterning...

I think things are much more complex than what the current definitions can define as Troy points out...

Xanthophores are the pigment cells responsible for producing yellow, orange, and red pigments. Erythrophores are the subset of xanthophores which produce red pigments. An axanthic animal should lack both reds and yellows. An anerythristic animal should lack reds, but may display some yellows, still.

-Cole

Actually the exact definition of axanthochromism is the deficiency or complete lack of yellow coloration... It says nothing about red... I would agree with there being a complete lack of orange as you would need yellow to produce orange...

I cut and pasted this... Not my own words...

Chromatophores produce their colors by reflection after absorption of light. Generally, the light comes from above, but it may come from below after reflection from an underlying structure. The most common type of chromatophore contains melanin (and is, therefore, often called a melanophore), which absorbs all wavelengths so that the chromatophore appears black; other types have red (erythrophores) or yellow (xanthophores) pigments. These pigments generally derive from carotenoids in vertebrates.

Going by this and everything else I have read, xanthophores are only responsible for producing yellow pigmentation...

are a sub-class of Xanthophores. Xanthophores are but one type of Chromatophore. True (= full) axanthism would result in the lack of yellows and reds.

Pigment production and deposition within each Chromatophores is the result of a biochemical cascade (like a chain of reactions). A malfunction at any step along any of the steps in these cascades can cause issues in the production or deposition of a given pigment. The Tyrosinase positive and Tyrosinase negative forms of amelanism are but one example (but note that most of our hypotheses regarding the identification of "T " and "T-" amelanism in herps have NOT been tested).

-Cole

note that most of our hypotheses regarding the identification of "T " and "T-" amelanism in herps have NOT been tested.

I have often times asserted that same fact myself and couldn't agree enough with you Cole!
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Troy Rexroth
Rextiles

Here's another link to a fairly well-done page explaining the basics of herp genetics: Herp Genetics.

-Cole

I know I did a complete breakdown definitive post on this earlier this year or last year but I'll be damned if I can find it using KS's sub-par search engine. So here I go again but this time in response to Gregg's query.

With the axanthics/anerys, are the hogs actually either of those???

I believe they are, and I believe they are Axanthic and not Anerythristic as some people keep labeling them as I'll explain shortly.

By genetic definition, Axanthics should produce no yellow coloration... With anerys, by definition, there is no red coloration produced...

This is pretty much true about how both of these terms are generally defined, unfortunately there are very few resources accurately defining the differences between the two. What we are left to do is look up the terms that are oppositely related such as Erythrism for Anerythrism and Xanthism for Axanthism.

Erythrism, as defined by Wikipedia and other sources is "...an unusual reddish pigmentation of an animal's body...". Anerythrism is just the opposite, so we are left to infer that Anerythristic animals lack red pigmentation. However, yellow pigmentation still exists and is often visible in phenotypic Anerythristic animals as well mixed phenotypic animals such as Snows in Corn Snakes. The 3 or 4 Snow Corns I've had over the last 16 years have always had some amount of yellow pigmentation generally seen on the neck but also other parts of the body as well.

Xanthism is defined as "...animals whose colouration is unusually yellow through an excess of yellow pigment, or possibly a loss of darker pigments that allows yellow pigment to be unusually dominant. It is often associated with the lack of usual red pigmentation and its replacement with yellow.". So, if Axanthism is the opposite, we are left to infer that yellow pigmentation is lost but that red pigmentation can also be reduced or absent as well.

With all of that said, Axanthism seems to fit what we have in the hognose world as I have never see any amount of yellow at all in any of the Axanthics in my collection nor in anybody else's. On top of that, I've never see any yellow pop up in the Snows either which is usually evident with Corns as they mature, but I can only go by what I've seen in the means of pictures of what BHB has and Vin Russo considering their Snows are a couple of years old now.

From what we see in the hogs, they do not produce either coloration which is obvious when looking at the belly where there should be orange patterning...

I disagree with this again based on the fact that Axanthism can exclude both red and yellow pigmentation from happening. The bellies of my 2 Axanthics, of which I've acquired in 2007 as babies have gone through several different color changes as far as lightening or darkening up during different growth spurts, but neither have ever exhibited any ventral coloration other white and black.

I think things are much more complex than what the current definitions can define as Troy points out...

I definitely believe this to be true with some of the morphs out there such as the Anaconda/Supercondas who seem to have linked genes of solid colored bellies when of normal coloration but are then lost when mixed with color morphs such as Albinos, PPA's and Axanthics (as so far observed) by going back to checkered bellies. Also the Red Extreme Albinos are a complex polygenic trait as well as all of the other linebred traits. But I think that the primary color morphs such as Albinos, Tpos Albino/Hypos, PPA's, Axanthics are fairly simple Recessive traits and aren't prone to too much variation in the general definitions of how their genes work. Even Toffees I don't consider 100% simple Recessive due to the "paradoxing" feature that is not understood. Personally I think the "paradoxing" with the Toffees is either a linked gene type of trait or that the Toffee gene is a faulty/leaky gene that is attributing to the amount of melanin that is being released. Of course that is all hypothesis based on things I've read about how some other genes/traits work. But I digress...
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Troy Rexroth
Rextiles