Do you think Isobel or John passed on he doppelganger gene(?) to Elena? I say Grayson as a possibility because I am not sure if it is explicitly said that it is only passed via the female line. I mean obviously only a female can be the doppelganger but do you think it could come from her father?
And another thing. Nadia Petrova must have had a child before she became a vampire. Why has nobody ever mentioned it?