American Fancy Rat & Mouse Association

This article is from the WSSF 2010 AFRMA Rat & Mouse Tales news-magazine.

Transgenic Mice & Tricolors

By Mike Chiodo “Mikiodo,” NY

Thoughts by Mikiodo, January 2007

I believe the tricolors/calicos are simply splashed mice with piebald spotting added. They originally come from transgenic mice from a lab. Of the originals I received, some were yellow splashed and some the standard grey/black. I later bred them to my pied mice and eventually got the so-called tricolors. The splashed coloration seemed to bunch up into more solid patches on pied mice. And some selective breeding made it more distinct. Wanda Wilson and I originally called these mice “genies,” short for transgenic, and that term referred to any of these mice with the splashes, smudges, or patches that came from the transgenic lines (not to be confused with Merles—which I had always called marked roans—and which I’ve read are genetically roan mice with an unstable roan gene). The naming can get so confusing!

So the formula for the tricolors I have is simply: two lower C alleles + piebald spotting (ss) + transgenic.

Below I have written a more in-depth summary of my thoughts on tricolors. If you or anyone else has any other thoughts, please let me know. Although I have a good understanding of mouse coat color genetics, there are many things I still have questions about.

Definitions of a few important terms I am using:

Transgenic: A transgenic mouse (or any other species) is simply an organism that has had DNA introduced into one or more of its cells artificially.

Chimera: An individual, organ or part consisting of tissues of diverse genetic constitution.

The original transgenic mice I received in January 1996 were from Wanda Wilson. She had received them from someone who had gotten them from a lab somewhere on the East Coast. Wanda said that no one was having luck breeding them. The first few I received were similar to what AFRMA calls Splashed. As adults, these mice developed swollen joints in their legs and looked pretty sad; I wondered if maybe they were being used to test arthritis or something? Anyway, I immediately crossed them to my best typey, healthy mice, and in a few generations they were as healthy and good-looking as my other mice. I have tried to adopt out as many as I could so that they would take a foothold in the mouse community.

Understanding the mechanism that creates these mice has been a little tricky. What I have surmised from some years of breeding them now is that the transgenic effect I have appears on any mice which have a lower-case allele at the C locus. In other words, Siamese, Himalayan, extreme dilution [Ivory], Stone, Chinchilla, Sable Siamese, etc.—these mice are ones you see the effect on. I have also seen it make a very subtle appearance on some lilac mice I have. I had once read in a scientific journal that mice which are Ccpp look markedly lighter than mice which are CCpp, even though C is supposedly completely dominant over its lower alleles. That would explain why I sometimes notice some subtle patching or splashing on my heterozygous lilac mice (Ccpp).

What happens is portions of the diluted fur revert back to the wild color, much like marked roans do (AFRMA’s Merles). I believe that is called chimerism also. So a Siamese genie mouse will have portions of fur revert back to the natural color (black); thus you get a Siamese mouse base with splashes or smudges of dark fur (chimeric fur which has reverted back to the black color).

Chocolate Tricolor
Chocolate Tricolor (note Siamese nose).

Whether this effect is dominant or recessive; well, I would maybe say semi-dominant, but I don’t know if that is accurate. I can breed a genie into a line without the factor, and if the offspring carried the lower C alleles, then it could be evident first generation. But since the effect is hidden on, say, a black mouse, then it would appear that it is not dominant in that mouse, even though it could actually be a genie mouse, but just doesn’t show the effect because it is not displaying the lower C alleles. I honestly don’t totally understand the mechanism to this day, so I can’t say for sure. And it has been pretty unpredictable.

I have found that once I add the piebald gene (ss) to the Splashed mouse, the areas of splashed darker fur tend to merge a little more, thus creating more of a patchwork appearance, not unlike Broken Merles. Typically, the more white a mouse has, the more solid the patches of color. So you have white spots from the piebald gene (ss), and the C allele color (e.g. Siamese), plus the areas that have reverted back (black). You get a tricolor, with beige, black, and white patches. Of course when you add blue or chocolate or yellow to the mix, you can get tricolors in those color schemes too.

I have a young male Logan who is a quad color . . . he is pied with a beige base, a black spot on his rear, and a chocolate patch on his head. I am assuming the chocolate color is just a patch of the reverted color black, which hasn’t gone all the way or something. Anyway, I will be breeding him soon to see if I can expand on that.

Tricolor (maybe quad)
Tricolor (maybe quad); white spotting is miniscule, with black butt spots.
Tricolor Logan
Logan. Quad color (hard to see colors here)—brown, white, off-white, black.
“Genie” babies
“Genie” babies with a Chocolate Tri and Quad boy.
Blue “Genie”
Blue “Genie” boy.

As far as the term “calico,” I would tend to use that as a separate term from tricolor, as it is a totally different mechanism. In cats, a calico is a tortoiseshell cat with white spotting. The mix of colors results in a cat with patches of yellow and patches of black fur (or chocolate, cinnamon, blue, etc.), plus the white. The size of the patches can vary from a fine speckled pattern to large areas of color (usually the more white areas, the more solid the patches of colored fur). This I would assume is the same with mice. I have many “English Brindles,” which either are sex-linked tortoiseshells or sex-linked brindles [Mobr] (which are an allele of tortoiseshell on the same X chromosome). English brindles are the base color with a brindling effect of lighter yellow or cream fur throughout the coat. I have many EB girls now in various colors and will be adding white spotting to the mix to see if I can maybe get some real calico-looking girls. These mice are almost exclusively females, as it is a sex-linked “lethal” trait on the X chromosome. Males do not survive; they die usually within 10 or so days, as they do not have a second X chromosome to compensate for the deficiency of the other. *

Fawn sex-linked Brindle
Fawn sex-linked Brindle female.
Lilac sex-linked Brindles
Lilac sex-linked Brindles.
Sex-linked Brindles
All of these are sex-linked Brindle girls in various colors and types.
Lilac sex-linked Brindles
Litter from pied English Brindle mom and pied “Genie” dad (all Long Hair). L to R: pied English Brindle “Genie”, pied Black, pied English Brindle, pied “Genie”, pied English Brindle “Genie.”

For more on Splashed (“Genie”) mice, see “Colors & Coats - Splashed Mice; “Tri-Color” Mice; Tri-Color/Calico Mice”

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Updated November 12, 2014