Black Unders genes all attach to the same area, creating a series. Creatures can only have two genes in a series at most, one inherited from each parent.
So they could, for example, be MaD (or) MaMa (but not) MaMa/D. They could also be nn for the series, and have no markings here.
Black Unders Series genes are codominant, meaning having more than one different type will show the effects of both.
However, having duplicates, like two copies of Ma, will not create two different types of Mask.
Black based, affected by any genes that target black such as merle
Black marking which may at minimum cover the muzzle, and at maximum stretch down the neck, shoulder points, underbelly, legs, and tail.
In some cases, can appear like a black version of Pointed
Skin is not affected. Eyes are not affected.
In hooved creatures, mane and tail are not affected.