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
Dun is a black pattern that appears on the back, head, shoulder, and legs. It may darken the affected areas only slightly or be completely black.
Dun factors are at minimum: A stripe of darker color down the spine and dark on the legs.
Dun factors may also include: Stripes of darker color going down the shoulderblade (which may spread to hips and tails), horizontal stripes from the elbows/knees down - or entirely dark legs from the shoulder/knee down, stripes on the tail, and stripes in a "diamond" shaped pattern on the forehead. The neck and tail may be darkened or ticked with darker color, this is more common in Barbary coated krowleus. Extreme cases my create heavy blotching over the topline.
Skin is not affected. Eyes are not affected.
In hooved creatures, it may create a streak of darker color on the inside of a mane/tail, leaving the outside edges paler.