In order to understand Axolotl genetics (characteristics expressed in Axolotls and their offspring), you must first have a basic understanding of genetics itself.
Genetics is the study of heredity and the variation of inherited characteristics.
In Axolotls the characteristics most commonly looked at are "colors". HOW an Axolotl appears visually is known as its phenotype (Leucistic, Melanoid, Copper, etc.). Phenotypes are determined by the animal's Genotype and its interaction with the environment. Before we dive into that, let's start with the basics.
Genes are units of heredity which is transferred from a parent to offspring and to determine some characteristics of the offspring (we are focusing on colors of Axolotls), which are proteins coded directly by genes. In simpler terms, a gene is a sort of blueprint for creating a protein (characteristic), and on a larger scale, an organism (Axolotl). There are about 30,000 genes on an Axolotl's DNA. We refer to all of an organism's genes as its genome.
There are DOMINANT and RECESSIVE genes. Dominant genes are complete blueprints for creating characteristics. Recessive genes are missing some parts or all parts of said blueprints. Because Dominant genes have complete sets of blueprints, they will always be expressed, opposed to the incomplete blueprints of Recessive genes.
All organisms begin life as a single cell referred to as a zygote. The zygote receives two sets of genes, one from mom and one from dad; one gene for every trait, or characteristic (skin color, blood type, etc). Every Axolotl possesses two genes for every single trait.
As breeders, we often use the term "het", or heterozygous. All this means, is that an organism has two different genes (one from mom, one from dad) for the said trait. Homozygous means the organism has two identical genes (one from mom, and one from dad) for a said trait. Going back to the term genotype, an Axolotl's genotype with two copies of melanoid is homozygous Melanoid, and it can be visually identified as Melanoid. An Axolotl with only one gene for Melanoid is now referred to as "het" Melanoid, and will not visually express that gene.
Now let's go in depth about Axolotl colors. The colors we see in Axolotls are produced by color cells, or chromatophores. These cells begin to form along the spinal cord and brain and spread across the rest of the embyro's body. This can actually be observed in developing eggs. Chromatophores of Axolotls are sorted into three different categories: Melanophores, Xanthophores, and Iridophores.
Melanophores contain dark brown/black pigments (eumelanins), produced by amino acid tyrosine. If melanophores don't function correctly, or are absent all together, different types of albinism occurs.
Xanthophores contain yellow/orange pigments (pteridine).
Iridophores (I know you have heard this one) are pigment cells that reflect light and appear as glitter/or shiny flecks on Axolotls.
Different types of genes control the color of Axolotls. These are: Albino, Leucistic, Melanoid, Axanthic, Copper, and GFP. We use those gene names to identify different colors of Axolotls in the hobby, among others.
Albinos, like we discussed before, have no Melanophores/have defective Melanophores. These animals do not have any melanin pigments in their skin or eyes. A normal Albino will have yellow pigments and iridophores, we call these "Golden Albinos". Albinos with complete absence of pigments are normally just called "Albinos", and have a white or pink appearance.
Leucistics are results of "leucism", the partial loss of pigmentation. It is different from albinism in that it results in the absence of multiple types of pigments, not just melanin. Leucism is caused by a defective or absent version of the gene that directs chromatophores to spread across a developing embryo. These Axoltols are normally pink/white with black eyes.
Melanoids have no iridophores whatsoever in their skin.
Axanthics do not produce Xanthophores, the chromatophores that produce yellow and orange pigments, therefore they will not have any. Juvenile Axolotls often have a purple hue to them.
Coppers cannot convert pheomelanin to eumelanin (contained by Melanophores we talked about above). These Axolotls are light brown in color because of this.
GFP refers to Green Fluorescent Protein, a protein that results in a green glowing Axolotl when exposed to black or ultraviolet light. This gene was taken from a fluorescent jellyfish and then inserted into an Axolotl's genome; this is an added gene, making it dominant (when present, it will always be expressed, or visually seen). GFP Axolotls are GMOs, or genetically modified organisms.
Let's talk more about Genotype, since it directly correlates with Phenotype, or color.
Alleles are different forms of a specific gene that occur at the same point (locus) on the DNA in a chromosome. Axolotls have 14 pairs of chromosomes and about ten times as much DNA as humans. Axolotls have a lot of copies of the same genes.
Different Alleles are represented by single letters; capital letters for Dominant genes and lower case for Recessive genes. There are FIVE pairs of alleles that will result in the primary color of an Axolotl, plus GFP gene.
D - dark d - white
M - non-melanoid m - melanoid
A - albino a - albino
C - non-copper c - copper
Ax - non-axanthic ax - axanthic
GFP - gfp gfp - non-gfp
Combinations of the five alleles result in all the different colors we see today in the hobby. There is no single allele for Leucistics or Golden Albinos, those are produced by different combinations of genes.
Now we will look at the colors and their specific genotypes, to give you a better and more thorough understanding.
Albino - a/a The "a" gene is HOMOzygous, and will be expressed, resulting in an albino Axolotl.
Melanoid - m/m The "m" gene is HOMOzygous, resulting in a Melanoid Axolotl.
Note, the genotype can be extensive depending on the genes possessed and represent more than just the color of the Axolotl's skin.
For Example, an Axanthic Copper - D/x A/x c/c ax/ax The "ax" and "c" genes are HOMOzygous resulting in a visual Axanthic Copper. "x" represents an irrelevant gene, "D" is the dominant dark gene, and the "A" is the dominant Albino gene. Neither are homozygous.
When an Axolotl is heterozygous for a gene, we will use axanthic for this example, it is represented as "Ax/ax". In order to produce visual Axanthic Axolotls, this axolotl must be paired to another Axolotl with the Axanthic gene (either homo or het for "ax").
*Wild type Axolotls possess all of the dominant genes, except GFP (but they can have that too). Wild Type - D/x M/x A/x C/x Ax/x gfp/gfp
Polygenes are genes whose individual effect on a phenotype, in Axolotls this would be color intensity, gill length and pigment, patterns, is too small to be observed, but can act together with other polygenes to produce observable variation. AKA, if you want to produce light Melanoids, pair light Melanoids together, or if you want long gills, pair parents with long guys.
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