08/29/2022
I have fielded a LOT of questions recently about “all-het” and “penta-het” axolotls. In particular, I get questions from people who are buying animals advertised with those labels, and want to know whether they are going to get what they are paying for.
Unfortunately, frequently they are not. That is a big problem because (a) breeders often charge a hefty sum for these special animals, (b) the buyer wants to use that animal for producing many varieties in a single clutch, or (c) the buyer wants to use the all-het to perform test-crosses with their stock to determine what genes they carry. If the animal is not what the breeder claims it is, then the buyer has been cheated and, worse, is going to be very disappointed or seriously confused (and most likely both) when breeding the animal a year or two later.
For those who don’t want to read this whole post, here is a short summary of the critical points:
(1) Breeding two all-het animals DOES NOT produce a clutch of all-het offspring.
(2) Specific crosses of homozygous animals are needed to produce all-het animals. If either of the parents is NOT homozygous then it is a crap-shoot as to whether any one baby is actually all-het or not.
(3) We need to stop using the term “all-het”, since it is generally misleading. Use terms like tetra-het, penta-het, hexa-het, etc for wild-types, and state the specific hets the animal has for ANY type.
Let’s start by defining the terms used here. “Het” is an abbreviation for the Genetics term “heterozygous”. A heterozygous animal has inherited different versions of a gene from each of its parents. If you slept through that lesson in Biology class, or have slept since then and forgotten, I’ve got a link to a basic Genetics lesson for axolotl nuts at the end of this post.
For example, if the s***m had a gene for albino and the egg didn’t have the albino gene, then the baby produced when that s***m and egg met is heterozygous for albino; It has both the recessive gene that causes albinism and the dominant gene that does not. An animal breeder will then say that this axolotl is het albino, or that it has an albino het.
If an animal has two identical copies of a gene it is called homozygous.
Because the albino gene is recessive, a het albino animal is not albino - it just carries the gene. An axolotl can only actually be an albino if both copies of the gene it gets from its parents are the albino gene; It has to be homozygous to be an albino.
Each animal gets one gene for each trait from each parent, so every animal should have two genes for each trait. (A trait is any observable feature of an organism, like color, tail length, blood type, whatever). In case you were wondering, an axolotl has many thousands of traits - about 30,000 pairs of genes. It is very close to impossible (except under very rare and unusual circumstances) for any axolotl to NOT be het for at least a few traits. Almost all axolotls are het for hundreds of traits.
A few years before I retired, I was designing a Genetics project for my students, and came up with the idea of producing animals that were het for all of the color genes then known. Up until that time, I had been teaching some of the basic rules of Genetics by having students perform simple test-crosses of some of my animals to determine whether they carried a particular recessive gene or not.
To do a test-cross, you usually mate an animal that you suspect is a carrier of a gene (is het for the gene) with another animal that is homozygous for it. For example, if you want to know if your copper axolotl is het for albino, just mate it to an albino. If about ¼ of the resulting babies are albino, then you know the copper is het albino. This is the way test-crossing is almost always explained to beginning Genetics students.
Unfortunately, that method is good for only one gene at a time. What if you think your copper axolotl is het for both axanthic and albino? How do you find out? With the method I just described, you would have to mate it to an albino, then wait a while and mate it to an axanthic. If you want to test for other genes, like white or hypomelanistic, you will need to do test crosses for each one. That is going to take a very long time, and you will have to have mature adults that are homozygous for each of those genes available to do all of those test-crosses.
A much better way to do a test-cross when you want to test for several recessive genes is to pair your animal with unknown hets with an animal that has ALL of the hets you want to test for! This way you only need to produce one clutch to identify all of the hets at once.
With this in mind, I paired a melanoid white albino female with an axanthic copper male. The resulting babies then got the melanoid, white, and albino genes from Mom, and the axanthic and copper genes from Dad. They were het for all five genes, so I called them “penta-het” wild-types (from the Greek word for five, pente). They were all “wild-type” because they only carried one copy of each of the five recessive genes.
Two years later, my students were able to use these penta-het animals to perform test crosses to identify ALL of the hets of any axolotl with a single test-cross.
For example, if you cross that copper from the earlier example with a penta-het axolotl you will get some babies that are homozygous for each of the genes the copper carries. If it carries the albino gene, then ¼ of the babies will be albino. If it carries the axanthic gene, then ¼ of the babies will be axanthic, etc. If ½ of the resulting babies are copper, but none are albino or axanthic, then the copper didn’t carry those genes at all.
In case you’re wondering; You can also pair two suspected penta-het animals to find out whether they really carry all five genes. If they do, then you’re going to get a clutch with every possible combination of those five genes - like the clutch in the photo accompanying this post. If you’re one of those people who learned to use Punnett squares to predict the results of that cross, be prepared to use a lot of ink or time in a spreadsheet. That Punnett square has 1024 boxes.
But if you were thinking that breeding two penta-het parents will produce penta-het offspring…
Yes and no.
Each baby only has ½ chance to inherit any one recessive gene from each parent. This means that for a baby to get the copper gene, for example, the probability is ½ that he will get it from Mom, and ½ that he will get it from Dad. Of course, there is also ½ chance he won’t get it from Mom and ½ chance he won’t get it from Dad.
This means that there are four different possible outcomes;
(1) The baby might get no copper gene from either parent, so it does not carry the copper gene at all.
(2) The baby can get a copper gene from Mom but not Dad, so it is het copper.
(3) The baby can get a copper gene from Dad but not Mom, so it is het copper.
(4) The baby might get a copper gene from both Mom and Dad making it a copper.
So there are two out of four ways to get a het copper from this cross. 2/4 = ½ chance that the baby is het copper.
This is true for each of the five recessive color genes. There is ½ chance of het copper, ½ chance of het albino, ½ chance of het melanoid, ½ chance of het white (leucy) and ½ chance of het axanthic. The probability of getting ALL of those hets, then, is all of those chances multiplied together: ½ X ½ X ½ X ½ X ½ = 1/32. Only one out of every 32 babies produced would get all of those hets, and, therefore, be a penta-het axolotl.
To put that in perspective, if a breeder produces a clutch of 100 babies (which is too many) from a penta-het pairing, only about three of the wild-type babies are going to be penta-het like the parents, and THERE IS NO WAY TO KNOW WHICH ONES THEY ARE!
The only way for a breeder to be certain that babies are really penta-het is to deliberately pair animals that are definitely homozygous for each of the color genes. For example:
White melanoid albino X axanthic copper
White copper melanoid X axanthic albino
Albino copper X white axanthic melanoid
Etc….
Unfortunately, some of those crosses are much easier to make with confidence than others. You can easily identify an animal that is albino or copper, but not one that is both. Unless the breeder has been very careful about their record keeping and breeding procedures there is no practical way to distinguish a white copper mel from a white axanthic, or an axanthic albino from a melanoid albino. I have been fooled by a melanoid albino that I thought was white (leucistic) but turned out to be genetically gold (I’ll put a link to a post about that one in the comments), and know of at least one breeder who made exactly that mistake when trying to produce penta-het wilds.
If you make the mistake of using an animal you think is penta-het (or whatever) for a test cross and it isn't really what you think it is, you've just wasted a lot of time and effort and now have no idea what genes your animals are really carrying.
It is also time to accept that penta-het animals are no longer the perfect tool for all test-crosses, and that the term “all-het” isn’t appropriate. Why? Because we have now confirmed a new color gene - hypomelanistic - and more are about to be discovered.
And let’s not forget all of the other traits we breed for, like GFP, RFP, Eyeless, Medusa, etc
Currently, the best test-cross tool is a hexa-het axolotl - one that carries all six of the (common) known color-influencing genes! (white, albino, melanoid, axanthic, copper, hypomelanistic) Unless your axolotl carries at least all six of these genes it is not justified to call it “all-het”, and as soon as the next new gene is confirmed even that won’t be acceptable. The addition of the hypomelanism gene to the mix makes correctly identifying which parents to use to produce those hexa-hets exponentially more challenging than producing penta-hets.
Many breeders (myself included) used to avoid revealing the method used for producing multi-het offspring because - let's face it - it takes a lot of work for us to do it correctly, and we don’t get paid if everyone knows they can do it on their own. But it’s been a while and that ship has sailed. Anyone who passed secondary-school Biology class should be able to figure it out by now. Also; I just explained exactly how it is done.
Please be careful and ask the right questions when paying big money for an all-het, penta-het, hexa-het, octa-het, whatever axolotl. The breeder should be absolutely sure of the genotype of the animal you’re getting, or should be clear and honest about the probabilities.
Link to Axolotl Genetics
https://docs.google.com/document/d/1kjdtH5JjnsXTe-IvbrT4kRnwCQTeHXTwmcSyUZtV4d4/edit?usp=sharing
