​http://www.horses-and-horse-information.com/horsenames/b.shtml

​A general rule that usually works well for marked rats.

Keeping in mind that a particular marking can be represented in a whole range of actual combinations (from way too much color to way too much white) there is a general rule of thumb.
Two rats with like markings (i.e., they are at the same level) can produce the full range of markings in their own level, and one level above.
Rats at levels next to each other will produce the range of markings represented by those levels.
Rats at levels separated by one level will produce mostly the level between them, but may produce small numbers of their own marking.
Rats separated by two levels or more will produce the levels between them, but will not produce their own level.
The levels are as follows:
1. Self
2. English Irish
3. Irish/Berkshire
4. Hooded/Variegated/Bareback
5. Capped
6. Masked/odd head spots/BEW
And yes, there are exceptions to every rule. This one will work in most cases however.