There are six known genetic traits that affect an axolotl’s pigmentation:
The Copper trait
The GFP trait
All six of these traits follow a Mendelian pattern of inheritance, which is good news, because it’s a very simple pattern to explain and understand. Trust me! Keep reading and you’ll be an expert on the topic in less than 5 minutes.
Mendelian inheritance: the Ikea metaphor
DNA is a pretty amazing thing: the complete set of instructions for the construction of one particular living organism. It’s often portrayed as one huge chain, but DNA is actually broken into individual segments called chromosomes. Think of each chromosome as one assembly instruction booklet, like the ones that come with Ikea furniture. Obviously it takes a lot of instructions to build a whole living being, so we need a whole pile of booklets.
The instructions inside the booklets also have to be fool-proof, because the ones reading them and performing the assembly are proteins, which pretty much work like mindless drones. This is fine, except that when the information in one booklet is messed up or missing, the proteins can’t pick up the phone and call Ikea for help.
Luckily, each instruction booklet comes in two copies: one that was obtained from the animal’s mother, and one from its father. So even if there is missing information in one of the booklets, the protein-drone just needs to look at the other copy. With any luck, the correct information will be there.
This is the basic principle behind Mendelian inheritance.
Let’s say I want to build a chair and I have two instruction booklets in my possession. Version 1 (which I got from my mom) shows detailed, step-by-step assembly instructions. Version 2 (which I got from my dad) has a bunch of mistakes in it, and it’s very confusing. If I follow version 1, I’ll end up with a chair. If I follow version 2, I might end up with some weird contemporary art sculpture that may or may not crumble when I sit on it. Obviously, I would rather follow version 1, right? I might call my dad up afterwards and tell him “Hey Dad, just so you know, the instructions you gave me made no sense! It’s okay though, I used a different set of instructions and I managed to build the chair in the end.”
But what if both of my parents had given me the faulty version 2? Since I’m not a mindless drone, chances are I would have gone “uhh, I don’t think this is right.” But if I were a mindless drone, I probably would take the fact that both sets of instructions are saying the same thing as a sign that the information is correct, and I would have built the weird contemporary art sculpture. And who knows, maybe the sculpture would have turned out even better than some boring old chair!
When I say that a particular genetic trait follows a pattern of Mendelian inheritance, what I mean is that the assembly instructions for that particular trait come in two different versions, and given the opportunity, the assembly protein-drone will always prefer one version over the other. The version that is always preferred is called the dominantallele. The one that’s used only if no other instructions are available is called the recessive allele.
Mendelian inheritance: the albinism trait
If a chromosome is like an instruction booklet, the section of the booklet that contains instructions for one particular trait is called a gene. Just like the booklet in our previous example, the albino gene comes in two versions: allele A and allele a. Dominant alleles are always represented by capital letters, whereas recessive alleles are always lowercase.
Just like humans, axolotls receive two versions of each chromosome — one from their mother and one from their father. Every axolotl either ends up with one of these pairs:
A/A (two identical copies of the dominant allele)
A/a (one copy of each allele)
a/a (two identical copies of the recessive allele)
Axolotls who end up with two copies of the dominant allele are said to be homozygous dominant. The ones with two copies of the recessive allele are called homozygous recessive. If they have one copy of each, we call them heterozygous (from homo = same, and hetero = different).
So what makes allele A the dominant version of the gene? It contains a set of instructions for the construction of melanophores, the pigment cells that produce the dark pigment eumelanin. In allele a, those instructions are either erroneous or missing due to a genetic mutation that randomly occured at some point during the evolution of the species. We call this mutation albinism.
Albinism works in a fairly straightforward manner: when an axolotl is homoyzgous for the recessive (mutant) allele a, it is unable to produce eumelanin (the brown/black pigment) because it simply does not have any melanophores. All other axolotls have melanophores and are able to produce eumelanin (with the possible exception of copper axolotls, which we will discuss later).
Even though albinism is a recessive mutation, it doesn’t mean that allele a is worse than allele A, or that albino axolotls are inferior in any way. Some mutations can yield positive results! Look at how cute these albino axolotls are:
Of the six mendelian traits that affect pigmentation, albinism is the most straightforward, because it only acts on one type of pigment cell. The other traits are slightly more complex, but the principle behind them is the same: as long the right sets of instructions are present, all pigment cells will be created and behave normally. But if they’re not, the assembly drones will follow whatever instructions they can find, and turn those functional chairs into pieces of art!
Female axolotls can lay up to 1000 eggs at once, which is exhausting for the female. She does not get a break to recover afterwards — her body immediately resumes gamete production, which comes with a high energy cost. For this reason, repeatedly beeding a female can be detrimental to her health. Breeding her too early can also interfere with her growth. Please be mindful of these considerations when choosing a female to breed — choose a female who’s fully grown (at least a year old) and has a healthy appetite and appearance, with a big round belly. Keep in mind that the same female should only be bred a maximum of three times in her lifetime, with a long break in between breedings. Personally, I try to breed females only once, unless they have exceptional characteristics. I also never breed females more than once a year.
Selecting a male
When it comes to choosing a male, the most important thing to consider is genetics. You’ll want to make absolutely sure that your male has no family relation with your female — this would lead to genetic defects in the offspring that can be quite dramatic. Beyond that, it helps to be familiar with how genes combine to create different morphs (phenotypes). Personally, I like to select males with traits that match the female’s best characteristic: for example, my “K” line is all about cute round faces, whereas my “B” line is all about blue gills.
Traits to avoid
You should never, ever breed axolotls with obvious genetic defects, such as:
any physical deformation that isn’t due to regrowth after nipping
a tendency to float frequently (especially upside down)
other recurrent health issues (e.g. very prone to fungus)
In case of accidental breeding
If you’ve accidentally kept a male and a female together and ended up with eggs, it may seem like the kind choice to keep them and raise them… But in reality, it’s the self-indulgent route that should be avoided in most cases. If the two parents are genetically related (e.g. brother and sister), or if one or both parents have genetic defects, you really wouldn’t be doing the larvae a favor by attempting to raise them. Not only would it compromise their quality of life, but it also poses a risk that the genetic issue will be passed on to future generations if those axolotls also end up getting bred (accidentally or otherwise).
Avoid this rookie mistake!
Another important point to consider is: how many of the eggs can you afford to keep? Raising larvae requires time, effort and space. They are also complicated and expensive to feed, compared to adults. If you are breeding axolotls for the first time, I wouldn’t recommend keeping more than 10. If you keep more than you are able to care for, you will be stretching your resources thin, and the quality of your care will suffer. Trust me — don’t try raising hundreds of axolotls on your first try. You have plenty of time to try your hand at raising more after you’ve brought these first 10 to maturity. You’ll be better prepared to tackle higher numbers once you have a clear idea of the challenges involved.
How to get rid of unwanted axolotl eggs
Freeze them. This will cause the larvae to go into hibernation mode, dulling their sense of pain before vital functions shut down. They will be unconscious before ice crystals begin to form. Once they are frozen solid, you can dispose of the eggs in the compost or trash.