My understanding was that IMG BPs are genetic but they don't pass on in the typical manner that recessives, co-doms, and doms will pass on. Like you could breed two IMGs together and you might get babies that carry the trait and you could breed those offspring back to the parents and you still may or may not get more IMGs. It always sounded like it's a random toss up to me which would explain why they aren't particularly common.