56% -- the majority of conscientiousness, it seems -- is not heritable. They can decide to, and be, but it sounds like a decent chunk of that is due to genetic predisposition.
Genetically I'm not much of a runner compared to my brother, but I work hard at it 3-4 times a week. But if he got around to hitting a treadmill as much as I do I'm sure he'd outperform me.