As yet no one is 100% sure.
In the US many breeders treat it as a recessive.
The problem is it takes so many years for some to change.
While I didn't see them hatch I picked this boy up from the breeder & saw both parents......both were visual Calico.
I'm no expert on this, Den has been working with these for years & would probably be the formative authority on Calico BHP's in Oz.
I can say I have a sibling to this boy & if he goes through the change I would be 'fairly' confident that the gene is simple recessive.
I think Den has had clutches that he has tried to keep tabs on & many have changed but not all.