I think it genetics to a point but I think anyone can get fat if they try hard enough. I don't think anyone is sedentary, eating a diet high in saturated fat and empty calories and walking the runway.
Habits are formed by the people who raise you which could easily make it appear that fat is genetic when really habits are passed down.
I learned to eat very well as a child. I had essentially zero processed or fast food until I was old enough to earn my own money and buy my own. That said, I fell in love with some of that processed crap and have to work at weight management now. I think had I kept an organic diet, I might be able to fool people into believing I'm a "naturally" thin person.