My knowledge is that there are at least 2 lines. Walter labels his SMR and McDonald, which apparently they are incompatible.
The 3 babies I have from Banshee x Big Sam are supposed to also be low expression pied, but I've not seen anything that indicates any expression. I'd have to search on here to find where the pied comes from within the Banshee and Big Sam lineages; I recall it mentioned at some point.
Just in observing pairings (adults) and subsequent offspring, it seems to me that the mutation itself behaves recessively, but the amount expressed leans possibly polygenic.
EDIT: something about bloodred/diffused being tied to the trait too....forgot that