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.
Simple black socks.
All four feet will be the same.
Edges may not be straight, may have a "wave."
May have black flecks along the edges.
Edges can reach knee, elbow, hip, and shoulder, but will dip in the middle in these extreme cases.