I think that the more 'general' aspects of personality are almost entirely determined by genes, while specific behaviors and preferences come more from experience or 'nurture'.
'Science' as we know it was invented by flawed organisms; while our minds may not be sophisticated enough (or perhaps willing) to objectively study ourselves, it is not necessarily impossible to do so at all.I believe that almost everyone who has ever broached this subject is talking out of their ass and truly have no idea. It's most likely a combo, but to quantify a person and everything about them with some catch-all theory is never gonna pan out.
I think genetics is responsible for the chemical parts of how we are as people (adrenal response, the balance of happy and sad chemicals in the brain, etc) but we learn how to act (personality and how we present ourselves as people) through nurturing.......
........To put it simply I think we learn things like acceptable behavior, bad and good, etc. from the people and environment around us, but instinctual responses are governed through genetics.
The jury's still out whether or not it's 100%, but apparently sexual preference is to a considerable part genetically predetermined. Aside from the scientific potential, just think of the one for trolling. You could organize an LGBT march to a Westboro Church branch under the motto "It's the will of God that we're gay/bi/etc."