that Jackson Avery is the chief's illegitimate love child with Ellis Grey.
Seriously, it kind of makes sense when you think about it. It would explain the chief's sudden return to his alcoholism (maybe the fact that the merger would place him in the same hospital as his son every day just pushed him to his breaking point?) and how the chief suddenly has taken a fatherly interest in Meredith (now this could just be because his drinking is reminding him of his relationship with Ellis and all, but it could also be because he wants to let her know that she has yet another relative in Seattle Grace and doesn't want to just be all "by the way, your mom had another child"). And Meredith's mom travelled so much that she probably could have just taken off to some random location when she started showing, had a baby, and claimed she was "working".