American Fancy Rat & Mouse Association

This article is from the Nov./Dec. 1993 AFRMA Rat & Mouse Tales news-magazine.

Colors & Coats

Reverse Siamese Mice; Torti Mice

By Troya Duncanson

Reverse Siamese Mice

From Wanda Wilson, New Cumberland, PA
Please discuss what we know about Reverse Siamese, what the genetics are, what colors they can come in. I have Reverse Siamese Frizzies with such small ears that they look like tiny teddy bears. All attempts to out-cross have fizzled out. Thanks.

A What was introduced as Reverse Siamese is a mouse that is homozygous for extreme Chinchilla dilution (ce) on a chocolate background. Unfortunately, the C series of genes is not known for even coloration, quite the opposite in fact. The C series of genes also can partially express with each other (unless dominated by a wild-type, full colored C). My personal goal is to breed an even-colored lighter shade that I can show as a beige mouse. But all of my English stock came from highly pigmented mice, so the uneven color is emphasized with very pale noses and rumps, and got dubbed Reverse Siamese. I don’t really know what cece looks like on a black mouse. I’m sure you’d get at least the same amount of lightening on nose and rump. I did get one Blue mouse with cece (I think). I ended up with a Black-eyed Silver mouse that was extremely mealy, not even at all, but very pretty. No noticeable lightening of nose and tail though. Unfortunately he died young and I never tried to continue with that color. Additional notes:
cece on a Black-eyed Fawn would produce a Black-eyed White.

cec (half albino, half extreme Chinchilla dilution) often shown as Black-eyed Whites, but usually show dark molt patterns. This is what AFRMA calls Ivory, Rat, Mouse & Hamster Fanciers (RMHF) show as Bone, and the National Mouse Club calls Cream.

chce - A Beige mouse with Siamese points (half Siamese, half extreme Chinchilla dilution)

c, ce, ch or cch can help take the yellow tinge out of a poor Tan (very poor tan) if the mouse carries one of these. My Chocolate Fox mice carry c, ce, or ch and have very white bellies when they’re carriers. It doesn’t seem to work as well for my Lilac Fox attempts.

You had asked what other colors Reverse Siamese comes in. Due to the drastic color dilution caused by the gene, I don’t think you’ll get a lot of variety or colors. Now the shades are extremely variable and you can easily select dark, medium, and light shades.

As far as out-crossing Frizzies goes, I’m sorry to hear things fizzled. I’ve had some nice typey short hair Frizzies pop up in a Variegated line—of course the hair straightens out shortly after they’re old enough to show! But I’ve had no problems getting Frizzy coats in a cross (F2 generation of course). But any selection for Frizzier coats is going to have serious effects on type. They don’t like to be typey mice.

Torti Mice

From Michael Emerson, Burnham, ME
I am writing with the hopes that you may be able to help us. That would be Michele Buck and myself with our endeavor to establish my Tortie mouse line. I guess I should give you a little background. April ’92 a female Tortie mouse was born in my mousery. The mother was a self black from my ELF line. The father was a self black from an ELF Siamese breeding. (ELF=Emerson linebred furless)

I bred Tortie Mouse to her father. She produced a Tortie female which I gave to Michele. After that she produced four more Torties—all black, red, and white; also all females. For one reason or another they all ended up dying before they bred. I then bred some of the female offspring from the Tortie ’s mating to her father back to the father. I got only one Tortie—a chocolate, cream, and white male. He died yesterday but never produced any Torties. Okay, before I forget, I bred the original female to the Tortie male several times. No torties were produced. In desperation I set up a breeding colony inspecting every single mouse produced—no Torties. Finally, last month I gave up and broke down the colony selling all but two pregnant females. One of the females produced a litter of ten with at least one Tortie—a chocolate, red, and white male. Now there may be some more Torties, only I’m not sure if they are Torties or Brindles. So now I received a letter from Michele and she has finally produced a Tortie which is black and white with a red patch on his face.

Here’s the problem. Unless it is or has a lethal effect, it is not a dominant. It is not, or I at least can’t see how it can be, a recessive as when I bred the two Torties together no Torties were produced, and the fact that we are getting both males and females leads me to believe that it is not a sex-linked gene as with cats. So now my planned course of action is to breed the Tortie I now have back to his mother and all his sisters to see what will turn up. I am sort of at my wits end desperate to establish this line. Any ideas that you may have would be greatly appreciated. I have written Bonnie Walters, but to date have gotten no reply. I thank you for your time and any help you can offer. LOVE-A-RAT.

A You don’t mention what your colony consisted of when you set it up, so I don t know anything about the pregnant female that produced at least one Tortie male.

Your plan to breed the male back to mother and sisters is sound. Either you or Michele should also try some crosses with unrelated stock (either Self Black or Spotted, depending on what you’re working with). Both to guard against fixing serious problems that may have accompanied this mutation, and to possibly find out if it acts dominant at all. (If it doesn’t act dominant, you’ll still have some unrelated mice to set up a second experimental colony with and see if Tortie pops up there.)

As this isn’t acting like calico in cats (and I don’t know cavy genetics) I feel free to suggest a possibility. (You ’re already familiar with dominant lethal, [which should still have given you more mice with this trait than you’ve got].)

Are you familiar with the term variable expression? A variably expressed trait can be either recessive or dominant, but not all of the individuals that can show the trait actually do so. Needless to say, this can be frustrating, as breeders of Odd-eyed or Tailless rats can tell you. You need to keep good records, so that you know which breedings produced the trait you’re looking for. Especially since siblings could be just as much of a carrier as the mouse that actually shows Tortie/Calico coloring. (Breeders of the above-mentioned rats are quite delighted when they have more than one rat in a litter that actually comes out looking like they want it to.) Anyway, variable expression is a possibility.

Watch out for genetic medical problems (which is why I suggest some out crosses too).

Finally, keep it up and please send me a photo! *

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October 27, 2018