Here you'll be able to find all the rules regarding the undercoat. The undercoat is not directly tied to any particular gene, and therefore it's an optional addition to any import regardless of their genotype, unless they present Epi White or Albinism.
The undercoat must always appear lighter than the base coat, but never light enough that it could be mistaken by white spotting.
On tabbies, the undercoat will go under tabby.
The MAXIMUM coverage for Undercoat is highlighted above.
While the undercoat may have such high coverage, it must NOT completely hide the original base color. If the base color isn't obvious, the undercoat may be accidentally mistaken for, or interpreted as, the base color and it could lead to corrections.
The undercoat can come in various shapes and designs. It can cover a large portion of the undercoat, or none at all.
Generally, smooth and even edges are strongly preferred, as they appear more natural! Here are some examples of good undercoat shapes.
When it comes to the edges of the undercoat, you can do any of these options.
Hard-edged
Soft-edged
Blended