Okay, so now you understand how the genes work, let’s talk about how we write them. Each gene sits in the same place in every mouse, so we can talk about the place if we give it a name. We use the latin word “locus” which basically means “place”, and we pick a letter of the alphabet for each locus. We use letters to write each allele, and we use capital letters for dominant genes, and small letters for the shy recessive ones.
Let’s start with brown. Brown is a recessive color, so we write it “b”, and you can think of “B” as “not-brown”. And when we talk about the B-Locus, we’re talking about the place that decides whether the mouse will have the brown dilution or not. If a mouse has the brown gene, it has to have two brown alleles because it’s a recessive, and we write the gene as “bb”. If it isn’t brown, the mouse could be BB or Bb, and we sometimes say the mouse “carries brown” because it has the gene but doesn’t show it. If the mouse isn’t brown and we don’t know whether it carries brown or not, then we write it either “B?” or B*” to show we don’t know what the other allele is.
Blue works the same way, it’s recessive, too, and for some reason (maybe because we already used b for brown) we use the letter “D” for not blue and “d” for blue. So a mouse in one of the blue colors has “dd”, otherwise it’s “D*” because it could be either “Dd” or “DD”. The D-Locus decides if the mouse will have the Blue dilution.
Eye color is black or pink, and we use “P*” for black eyes and “pp” for pink eyes, and we talk about the P-Locus for eye color.
The genes for self and ticked and tan all start with “A”, and these colors are decided at the A-Locus. Ticked is dominant, and the basic color is Agouti, so it’s an A. A self mouse is written “aa”. Tan is actually written “at”, and you can have self or ticked tans, and an agouti tan would be atA, a black tan would be ata or even atat, and a double-tan sometimes has a redder belly. Red is a special gene, written Ay, and it’s dominant over all the others. Ay* is always red (or fawn or buff) no matter what the other gene is. You can even have an Ayat mouse, which would be a red tan, except that a red mouse already has a red belly so you can’t tell. If you get certain genes for shading, that same mouse with Ayat gets a darker top shading to a redder belly, and it’s called a Sable. Brindles come from the Avy gene, which is dominant over everything but red.