It's mostly genetic but also how you use your hands can have an effect in its development.
An example of this is this one young woman, 15 years old, who has been climbing trees with her pet monkey since she was four. At age 13, she entered her first rock climbing competition and beat the previous top rated champion who's been climbing professionally for years. She had not climbed indoors until she was 13.
Anyway, there was this show on cable TV that used science to explain amazing things that happen to people so that people can do. She was one of them and she was probed to figure out just how she climbs the way she does. They measured her and what struck me were her hands. She had very nice hands in the functional sense but her fingers, the doctor said, were extremely long (they seemed normal length to me). Her knuckles and finger joints were larger than normal to support her weight while climbing the doctor said. Her arms and legs were very long, too. Her armspan was abnormally long, too, at 2" longer than her height. All of these physical traits are traits that shd would not normally have if not for her years of climbing trees as a child.
I also remember her specifications that read out on the computer screen.
Height: 5' 3"
Weight: 110 pounds
Body fat: 16.3%
Armspan: 5' 5"
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So how you use your body has a direct effect on its development. Until you get too old and your growth spurt stops. :-/