I believe femininity and masculinity are both social constructs. You are not born masculine or feminine. As you grow up you are conditioned to act the way you do.
Its both. To a point you are born with certain traits that are in your genes but there are also the fact that we learn by imitating whats around us and there for nurture comes into affect.
Like the story about the twin boys that were born and with one there was an accident when they were getting circumcised and the doctor suggest they turn him into a girl, they raised the child as a girl, but they always wanted to be a boy (obviously the story is more in depth, that's just my brief version of it). It kind of showed that despite being raised and treated as a girl, nature was that they were a boy, and they knew it deep down even though they weren't told for year what happened.
I think both obviously come into play, they have to. I also think it's different for every person.