Guo Jing, Yang Guo, and Zhang Wu Ji are people who come from humble backgrounds, speaking purely in martial arts. Guo Jing and Yang Guo's families were long revered to be heroic and patriotic, but Guo Xiao Tian, Yang Tie Xin, Yang Kang, and Bao Xi Ruo weren't even middling fighters, or fighters at all. Zhang Cui Shan and Yin Su Su were at a higher level of martial arts, but neither could be considered a top-tier fighter in The Heavenly Sword and Dragon Saber.
Because of this, I think we could rule out a genetic component, but what exactly does it take to become skilled in martial arts? Does it take a certain kind of person with a solid foundation, or is it being able to learn formidable arts and then years and years of dedication and refining their skills?