Assumes Correlation and Causation, ignores Social and Economic situations of the twins groups, doesn't account for the fact that identical twins tend to share (being geared towards) more experiences due to being perceived almost as a single "being", ignores cultural differences in upbringing of different-gender twins, doesn't account for indirect training (i.e. strategy, memorization, sequential steps etc. are parts…
The importance of right genes is pretty clear when you look at sports. Different sports favor different body types and if you don't have one, you won't suceed.