Phew! That’s a lot of letters. Are you seeing spots? The white-blob type of spot is a recessive gene, and we use S* for not spotted, and ss for spotted. You can figure out if the mouse has spots that it is ss, but sometimes a mouse can have the spot gene but not show any spots. Sometimes you can even get a mouse that is all spot with no color!
Some mice have white spots in big or small blobs, and these are called dutch, or evens, or brokens, or just spotted, depending on what pattern the spots take. The gene for the white blotches is a recessive, so both parents have to have it somewhere for the babies to have spots.
There is a dominant type of spotting that creates the look we call “variegated” which is harder to describe, but there are pictures at various sites. It can look kind of like the mouse had paint dripped on it and somebody tried to rub it off, but some is still on in places on only some of the hairs. You get a kind of smudged effect.
For the dominant spot gene we use, well, we use “W”, and I have no idea why that letter was picked. If a mouse has the W gene, it should have the funny smudgy spotting, except once again, sometimes there are other genes we don’t keep track of that can make the mouse not have spots at all.
Another thing about the W gene: it’s always either Ww, which is a mouse with smudgy spots, or ww, which is a mouse without smudgy spots. If there were going to be any WW babies, they don’t live long enough to be born, because there’s some problem with the gene. The same is true for the Ay gene for red mice—the AyAy babies are never born. So if you have two red parents, or two smudgy spotted parents, the litter will be a little smaller because probability predicts that about ¼ of the babies will have that lethal combination of AyAy or WW.