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Axolotl Genetics for Dummies

This is a discussion on Axolotl Genetics for Dummies within the Axolotl General Discussion forums, part of the Axolotls (Ambystoma mexicanum) category; Genetics can be confusing when there's more than one gene involved. The genetics page on Axolotl.org is great, but it ...

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Old 29th January 2016   #1 (permalink)
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Arrow Axolotl Genetics for Dummies

Genetics can be confusing when there's more than one gene involved. The genetics page on Axolotl.org is great, but it can be a little confusing. My intention with this post is to break it down and give a little more background so it's easier to understand what's actually going on with the different basic colors.

I'm going to start with the basics. Skip it if you've already got it all.

One gene can have multiple alleles (versions of the gene). "Genotype" is the alleles that they have (e.g. A/a) and "phenotype" is the manifestation, what we can see, that results from their genotype (e.g. Wild type or albino or melanoid) For example, humans have different genes that make up eye color, but it can be simplified (oversimplified, but easy to understand) for the purposes of discussion to one basic gene with a couple different alleles: brown and blue. Brown is the dominant allele; blue is the recessive allele. Homozygous means two of the same alleles; heterozygous means two different alleles. People will have brown eyes if they are homozygous (B/B) or heterozygous (B/b) for the dominant brown (B) allele. A person might have a genotype of B/b and a phenotype of brown eyes. They will only have blue eyes (since blue is recessive) if they are homozygous for the recessive blue allele (b/b). In real life, it's multiple genes, and that's why it's not JUST brown or blue. There's green, there's hazel, there are totally different shades, etc. But the basics can explained with eye color. One gene can have two or more alleles.

Axolotls have 4 (or more) genes for coloration. Wild type means they are either homozygous for the wild coloration or heterozygous for one wild allele and one mutant allele (theoretically, all colors other than wild type originated from a mutation at some point). A wild type axolotl will display the wild type coloration (brown, green, yellowish, even almost black), but they can be carriers for albinism, melanoid, and leucistic. My wild type axolotl, Titus, has a wild type phenotype, but he is heterozygous for copper (a color that I will not talk about since I don't think most of us, certainly not me, know that much about how it happens). He has a wild type phenotype, but he has a heterozygous wild/copper genotype. If I were to breed him with a copper female, his offspring would theoretically be half copper and half wild type.

Referencing the Axolotl.org page, D is dark, M is non-melanoid, A is non-albino, and AX is non-axanthic. The D gene makes them dark obviously, with brown/black coloration. The M gene is associated with iridophores, which make them iridescent or shiny. The A gene is associated with true albinism, a lack of pigment, not just white bodies and pigmented eyes like a leucistic. The AX gene is associated with xanthophores, which has to do with yellow and red coloration.

A wild type axolotl is either homozygous or heterozygous for the dominant version of each of those 4 genes. D/D or D/d, M/M or M/m, A/A or A/a, and AX/AX or AX/ax.

It doesn't really matter whether they're homozygous or heterozygous in terms of their own appearance, but it does make a difference for what they'll pass on to offspring. A homozygous animal can only pass on what it has. A heterozygous animal can pass on one allele or the other. So a wild type that is heterozygous for A/a, when bred with an albino axolotl (a/a) would have approximately half wild type A/a and half albino a/a babies. This is because the albino parent would only pass on the albino allele (a) and the heterozygous wild type parent would pass on the 'A' to half the babies and the 'a' to the other half. Two heterozygous parents, for example two wild type axolotls that are both heterozygous for albinism (both A/a) will produce approximately 3/4 wild type and 1/4 albino (Half will be A/a--heterozygous wild, 1/4 will be A/A--homozygous wild, and 1/4 will be a/a--albino).

SO... breaking down the different color morphs...

As previously said, a wild type has either homozygous or heterozygous dominant alleles. They may be very dark brown if they have lots of D pigment and not much AX (red/yellow) pigment, or they may be more green if they have more AX red/yellow pigment to combine with the D brown pigment.

A melanoid will be m/m -- homozygous for the recessive gene that affects iridophores (shininess). They will always be m/m. Melanoid axolotls lack the shiny pigments that reflect light. Their eyes will not be shiny like a very dark wild type's might be. (My wild type axolotl, Titus, is very dark brown, but his eyes are a shiny copper color). They tend to have lots of dark pigment (D) and not much of the red/yellow pigment (AX), so they are USUALLY very dark, even black. This is certainly not always the case. Plenty of melanoids are grey, even light gray, and some can even be albino since that's a totally separate gene! Even melanoids that are black can be heterozygous carriers of the albino gene and could produce albino babies if bred with an albino or another heterozygous carrier of albinism. A melanoid will always be m/m, but it can have any combination of the other genes that affect a color (e.g. a melanoid can be dark and non-iridescent since it is m/m but might also be A/a, D/d, and AX/ax, therefore it could produce pretty much any coloration depending on the genotype of the axolotl it's bred with). The exact coloration of the melanoid will depend on the 3 other genes. There can be melanoid albinos, very dark melanoids, light gray melanoids, (gfp melanoids), etc. Breeders try to produce homozygous individuals so the offspring are predictable!

A leucistic axolotl is not albino. The root word "leuko" just means "white." It's a totally different primary gene. Leucistic is probably the most specific overall phenotype. As explained in the Axolotl.org page, they are leucistic because the d/d genotype prevents the dark pigment from migrating across the body when the embryo develops. They are white or pink, yet they have black (or sometimes blue) eyes. A leucistic will always have a d/d genotype, and it also must be either A/A or A/a (non-albino).

Albino axolotls come in a few different varieties. All of them are a/a (they are homozygous for the recessive albino allele on the albinism gene). They are albino because the albinism gene blocks the dark pigment (D) gene. They all will be fairly light in color and have white or pink eyes. However, their appearance can be different depending on their other coloration genes. A very common morph is golden albino. They are albino due the a/a albino genotype, and they are golden because they are either AX/AX or AX/ax, meaning they still have the red/yellow coloration from the xanthophores. They can be D/D or D/d, but the dark pigment is blocked by the albinism. White albinos and axanthic albinos look almost identical (they both look very light or white), but white albinos are considered to be a/a and d/d while axanthic albinos must be ax/ax, so any dark pigment will be blocked by the albinism and there won't be any yellow pigment since they are homozygous for the recessive 'ax' that blocks xanthophores. Lastly there is melanoid albino. This just means they are m/m--melanoid--(no iridescence from the iridophores) and they are a/a--albino. They still might have a little bit of yellow coloration if they are AX/AX or AX/ax. A yellow albino, white albino, or axanthic albino might have shiny eyes, gill stalks, etc., but a melanoid albino will not have any of that shininess since that is what the melanoid gene blocks.

The phenotype of an albino can make it hard to determine the genotype. That's why you won't usually hear very much about axanthic albinos or melanoid albinos. They may look very similar to a white albino or even a golden albino. The way to know for sure is to know the the genetics of the parents, and even then it may be difficult. If your axolotl is yellow with pink or white eyes, it's probably a golden albino. If it's white or light pink with pink or white eyes, you can just call it albino unless you want to get fancy.

I'm not a geneticist. I DO have plenty of background with genetics (and science in general) since I'm in my last year of pharmacy school, but I'm not an expert. I tried to break this down though, after looking at a lot of it over and over and thoroughly referencing John P. Clare's page on Axolotl.org.

If you read this whole long-winded post, I hope you found it helpful!



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Old 29th January 2016   #2 (permalink)
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If anyone notices any mistakes, just let me know and I can fix it :)



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Old 25th May 2016   #3 (permalink)
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Very helpful thank you! My axy's have just had their first batch of eggs and me being me (think it's the school teacher thing) I need to learn everything I can :)

Most of all I found this a great refresher! High school human biology / biology classes were a long time ago haha

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