If you've wondered why some people just take naturally to some competitive sports or exercise than others, the answer may be in their genes -- but that doesn't mean genetics is the only thing at play. Some people respond to training better than others, but everyone can be trained. This video from ASAPScience explains why.
The question of whether nature or nurture is more important in the development of physical abilities isn't going to be resolved by a single video, or even a single set of studies. What we do know is that nature definitely plays a role in how well you can pick up a physical skill, like cross country skiing, marathon running or any other endurance sport. Some people have a higher baseline endurance than others, and people related by blood tend to train in similar ways. That means that if your father or mother was a great endurance runner, you may have the genetics to take to it as well.
However, even if you don't, and even if you're not the type that just takes to physical activity, all isn't lost. The differences really start to appear at the competitive and high level, but the same research showed that everyone benefited significantly from training, and no one just "couldn't do it because they didn't have the genes for it". In the end, some people may have a leg up, but everyone can learn with the right help, so don't give up.