Do Ballerinas have thin body frames because of genetics or because of Ballet ?

I have noticed that people always have the body of their sport.

Like for example girls who practice martial arts, have bigger bodies, and Ballerinas always have an extremely thin frame.

I am not talking about their weight, i mean that their whole bodies is far thinner (bones skeletal structure, all that).

So I am wondering if they have the body they have because of the sport practiced from childhood and that their bodies developed like this because of this.

Or if they were just born with these body types / body frames (extremely thin skeletons) and therefore chose their sport accordingly.

I really don't know. Thank you!

EDIT: I should specify that i wasn't speaking of Professional Ballerinas. I have seen plenty of actresses for example who did Ballet as a Hobby, and the frame of their Body is So thin like a ball jointed doll. (Again I'm talking of their shape when i say frame. Not their weight.) So i wonder if they were born this way or if it is thanks to Ballet. Thanks everyone for the answers!♡