[Cait]

I disagree that a/at tans all look the same as at/at. Although when talking pets they can both look the same, I would say that in show mice you'd see a difference, because the tan wouldn't be as strong on a/at, especially over many generations. That's why you breed tan to tan and don't put other things in unless you have to try and get rid of tan creep or drastically improve top colour etc.