I say "general rule" because there will always be well connected guys who might get made even though they aren't 100%.
Of course, it's more common if the father is a made guy himself. Greg DePalma's son, Craig, is fully Jewish on his mother's side, yet he was made back in the very early '90s (and at the young age of 25, to boot). Although, he hasn't had a very happy ending. He's been in a coma due to a botched suicide attempt in prison back in 2003. His father died in prison back in November, a virtual pariah due to the "Jack Falcone" fiasco.
