@crucker2 Yep, this is occam's razor in a nutshell. Of course spontaneous mutations and chimerism, but they are incredibly rare compared to having a side-piece.
That said, O is recessive, A&B are equally dominant.
So O+A=A, O+B=, O+O=O and A+B=AB.
BONUS: Mum freaked out and thought I somehow...