5. at - Black-and-tan

at is the highest dominant of the a-locus recessives. The basic action of this gene is removing the yellow/brown band of the agouti type in the hairs on the dorsum of the mouse (leaving the top black) and making the belly tan (or yellow). There is a demarcation line running horizontally on the sides of the mouse from the nose to the tail root. The demarcation line runs rather low on the mouse, from just above the jaw, along the sides. The top sides of mouse's legs should be of the top colour, while the insider of the feet should be tan.

It is an interesting gene, with differences in dominance and recessiveness on the top (dorsum) and belly (ventrum) side of the mouse. According to Mouse Genome Informatics, at is dominant to all a-locus genes on the belly. The exception to this rule is noted as being Aw, alhough it's said that one simply can't see the difference between an agouti with at and a Aw mouse. On the top / back of the mouse, at is recessive to all higher a-locus alleles (A to Ay). When it comes to lower a-locus recessive, at is fully dominant on them, both on dorsum and ventrum.

Being dominant on the belly and recessive on the back means that at determines the belly colour of the mouse, no matter what the other a-locus gene is. However, the colour of the mouse's back is determined by the other gene present: if it is higher on the overall dominance scale than at, the back will be coloured according to the effect of that other gene. If at is paired with a gene lower on the dominance scale, the back will be black.

Although at makes the back of the mouse black instead of agouti, it doesn't make the mouse's back completely black: there will be yellow (tan) hairs behind the ears and in the ears (pinna hairs). Selective breeding is needed to reduce the amount of these hairs as much as possible. Furthermore, the overall colouring of the belly is dependant on the genetic background of the mouse: some have all-yellow (all-tan) bellies, while others have tan with black hair roots.

Black-and-tan gene is an old mouse fancy gene and one of the original varieties at the foundation of the National Mouse Club in 1895.

Varieties with at include: agouti tan, cinnamon tan, black tan, chocolate tan, blue tan, lilac tan, dove tan, champagne tan, silver tan. There are also chinchillated at -varieties black fox, chocolate fox, blue fox, lilac fox and extreme dilution foxes bone fox and beige fox. at is also present in chinchilla and argente creme (exl. those of the Aw lines). at is also responsible for white bellies on pointed varieties.

5.1. Homozygous forms

Noddyn Tina Matleena

ba/t, at/* B/* C/* P/*
pic: Arttu Väisälä

All varieteties showing the effects of at excluding the agouti-based tan varieties (A/at) can be either homozygous (at/at) or heterozygous (at/*). Both homozygous and heterozygous tan varieties can have very good, fiery tan bellies - and both can have poor, faintly yellow tan.

5.1.1. Black Tan

Black tan is genetically at/* B/* C/* D/* P/*. Thus, it is the "basic tan variety", with all the other "self-based" (that is, with non-agouti top colours) tans being dilutios of black tan.

For a breeder of black tans or any other tan variety, it is important to understand, that the fiery tan belly called for in standards isn't what the gene gives "automatically". This belly colour has to be selectively bred for. Without selective breeding, the belly colour will be nothing like the standard's description. It will be of faint yellow colour, rather light. It can also be poor enough to look like a very poor fox belly.

As has been already noted above, at does not remove all tan hairs on the back of the mouse, as tan hairs can be found behind and on the (insides) of the ears. tan ticking running along the sides above the demarcation line is also common. Sometimes, with tan mice of a very good tan belly, the tan tends to "creep" up the sides of the mouse: up the cheeks, around the ears and over the tail root. The feet and legs of these mice also tend to be wholly tan.

Rapunzel's The Goldberg Variation

Agouti tan, A/at B/* C/* D/* P/*
pic: Anniina Tuura

Another common problem is found on the belly of the mouse: throat spot sporting the mouse's top colour. This can be bred out, but usually at the cost of the depth of tan.

5.2. Heterozygous forms

5.2.1. Agouti Tan

Due to at's effects, only agouti tan and its dilutes (cinnamon tan etc) are necessarily heterozygous.

A/at heterozygotes show the interesting partial dominance of at over A: the gene doesn't have any effect on the dorsal colouration of the mouse, but it dominates the ventral colouring.

5.2.2. Black Tans and Other a-based Tans

As tan dominates over a, all non-agouti tans can be either at/at or at/a. The depth of tan belly is affected by other genes present; the cordovan dilution in chocolate tan, blue dilution d in blue tan and both of them in lilac tan. Furthermore there's the pink eyed dilution working in the pink eyed tans. Modifiers have a major say in shaping the final shade of the mouse.

5.2.3. Foxes

Most standards include the following Fox varieties: black, chocolate, blue and lilac. Fox is, of course, possible in the pink eyed versions of the varieties mentioned, but they are first of all difficult to achieve due to the linkage of C & P loci, secondly they most likely won't look like much - being so light.

All the foxes mentioned have one thing in common: they are chinchillated fox varieties. That is, black fox is at/* B/* cch/cch (D/* P/*).

There are other genes diluting the tan belly into fox. These other genes are also situated in the c-locus and are himalayan (ch), extreme dilution (ce) and the heterozygous combinations of them.

Quiz Time!

Quiz two: Why there can't be siamese tans?

Answer: Genetically speaking a siamese tan is possible, just introduce the tan-gene at into a siamese mouse. Phenotypically speaking, siamese tans are an impossibility. While a mouse can be genetically a siamese tan, its phenotype will be that of a foxed siamese. This is because the siamese gene ch dilutes the tan belly into fox.