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I recently saw coastal carpets described as 100% het tiger. It's my understanding that the tiger gene is co-dom, and the only other mention I've heard of the term "het" to describe them has been Anthony Caponetto's reference to his first generation tigers (tiger X normal). His snakes were obvious tiger offspring, which he commented may be visual hets due to the broken striping, which when bred together may produce "textbook" tigers.
The other coastals I saw looked perfectly normal, w/ the exception of some increased side striping, and a tiger-like head pattern. They are reported to be the offspring of a normal X tiger pairing, all of which had the same visual appearance.
Does anyone have any thoughts or theories about this? Could this be the result of pairing a first generation "visual het" to a normal? Thanks.

"Heterozygous" or "het" simply means that the two genes in a gene pair are different. They could be a normal and a mutant gene or, in the case of multiple alleles, two different mutant genes. There is no requirement as to what a heterozygous animal looks like in that definition. In other words, a het tiger carpet python has a tiger mutant gene paired with a normal gene. What that het tiger looks like has nothing to do with the fact that it is a het tiger.

The heterozygous animal's appearance does determine whether a mutant gene is considered to be dominant, codominant, or recessive to the normal gene. Here are the ideals:

If a mutant gene is a recessive, a snake with a normal gene paired with the mutant gene looks like a snake with two normal genes.

If a mutant gene is a dominant, a snake with a normal gene paired with the mutant gene looks like a snake with two mutant genes.

If a mutant gene is a codominant, a snake with a normal gene paired with the mutant gene looks like neither the snake with two normal genes nor the snake with two mutant genes.

Those three categories are human inventions. Sometimes nature gets sloppy and doesn't exactly fit a category. Then it's like picking a pair of shoes -- none fits perfectly, and the choice comes down to picking the best fit available.

The best way to be get animals that are known heterozygous is to mate a snake with two mutant genes to a snake with two normal genes. All of the babies are then heterozygous (AKA 100% probability heterozygous). If these babies do not look normal but also can be picked out from snakes with two tiger mutant genes, the best fit may be "codominant", with variable expressivity producing a variety of more or less intermediate appearances in heterozygous animals.

Let's say that a "classic" tiger has two tiger mutant genes and a heterozygous tiger (one tiger mutant gene paired with a normal gene) has a tiger head pattern and some broken striping. And a snake with two normal genes looks normal, without the tiger nead pattern or any striping. Then expectations for the various possible mating are as follows.

"classic" tiger x "classic" tiger -->
all babies "classic" tiger

"classic" tiger x heterozygous tiger -->
1/2 "classic" tiger
1/2 heterozygous tiger

"classic" tiger x normal -->
all babies heterozygous tiger

heterozygous tiger x heterozygous tiger -->
1/4 "classic" tiger
2/4 heterozygous tiger
1/4 normal

heterozygous tiger x normal -->
1/2 heterozygous tiger
1/2 normal

normal x normal -->
all babies normal

In other words, if there are no normal babies, then the most likely explanations are 1) at least one parent has two tiger mutant genes, or 2) it's a small clutch of eggs and the luck of the draw was against you.

Hope this helps.

Paul Hollander

Thanks Paul-
It helps somewhat. The only thing I'm having a hard time figuring out is why I've never heard of the term "het" to describe the tiger gene before. Much like the Jaguar gene, a carrier displays it in the first generation. Non-carriers from the same clutch appear and are "normal". I also remember reading that either the Jag or Tiger (don't remember which or if it is both) have no heterozygous form. I may have to spend a little $$ to really figure out if these are actually "het", and how the gene works. Thanks for your help, and any further insight is appreciated.
Mike

The reason you've never heard het applied to the tiger gene is the general ignorance of standard genetics terms among herpers. For many years there were only recessive mutant genes, and people got the impression that heterozygous only applied to recessive mutants and that hets look normal.

Then mutants that were not recessive started showing up. Somebody started defining codominant as having one of these mutants paired with a normal gene and not looking normal. In standard genetics, such a snake would be called heterozygous for a dominant or codominant mutant gene. And dominant got defined as having two of these mutant genes (standard genetics = homozygous for a dominant or codominant mutant gene). Dominant and super often mean the same thing in this dialect.

In my opinion, having this sort of ghetto dialect get established is one of the worst things that can happen to herp genetics.

Paul Hollander

Thanks, and can you recommend any good books on snake genetics? Also, those who have been working w/ Carpets for years have had a difficult time figuring out and/or explaining exactly how the Jaguar and Tiger genes work. I have seen and heard it referred to as poorly understood on more than one occasion. Can you give any insight to these specific morphs, and if the genetics of these morphs are so poorly understood, would publications on general genetic help? Thanks.
Mike

I haven't seen all the books that touch on snake genetics. The best one I've seen is on corn snakes, but it doesn't discuss dominants or codominants. So, in my opinion, you are better off going to a general genetics text. Schaum's Introduction to Genetics, by Elron and Stansfield, is better than most though has some faults. I like the problem-based format, and used paperbound copies of the latest (2002) edition are available for less than $10.

I know nothing about either the Jaguar or the Tiger morphs. Most of the genetics I've done has been with mice and ringneck doves. But the genetic principles are the same in pythons, mice, corn, etc. So I will look around on the web and see what I can see. The only thing that will really help work the problem out is a pile of detailed, multigenerational breeding records.

Paul Hollander

Did a quicky web search. Below is the most interesting site I looked at. It had pictures from three tiger x normal matings.

You can almost guarantee that web site information is incomplete. So I'm going by the pattern that it shows. Like calling a colon followed by an end parenthesis a face that is smiling.

Apparently, mating a tiger to an unrelated normal snake produces all tigers. Or at least all the babies are not normals even if not good tigers. This is consistent with a snake with two tiger mutant gene mated to a snake with two normal genes producing all babies with a tiger mutant gene paired with a normal gene. And I kept tripping over the word "variable". There seems to be enough variation in the babies that often you can't tell whether a snake has two copies of the tiger mutant or one.

So, in my opinion, the information for tiger best corresponds to a dominant mutant gene with variable expressivity. That opinion may change some with more information about the variablility. But I'm pretty sure tiger is not a recessive mutant gene.

As for the claim that tiger is a codominant, I've already given my opinion on how poorly most herpers understand the term.

I found less information on jaguar than tiger. Very tentatively, I lean towards the opinion that jaguar is also a dominant mutant with variable expressivity.

Paul Hollander

Anthony Caponetto Reptiles - ACReptiles.com

Thanks Paul-
Ironically, Anthony Caponetto's site is the one that actually sparked my interest in Tigers to begin with. Information found there is also what caused me to question the "het" tigers which looked about 95% normal. Thanks for the book recommendation also. I think I'll check it out, and in the meantime, I'll do what best sums up my life.... jump in feet first, and see what I can make happen!
Mike