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 dominant allele. 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!
<- Axolotl Genetics, Part 1: Color Pigments | Axolotl Genetics, Part 3: Melanism and Axanthicism -> [Coming soon!]