system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do trees determine where to branch, and is it more dependent on DNA or environment?
gpt
Photosensitive growth hormones. 

Just like your body uses chemicals such as testosterone to tell it's cells when and how to grow, trees and plants have their own equivalents, only some of the plant-hormones react to light. 

The hormones that decide where a new branch start are (in some trees) more active when exposed to light, while the ones that decide which side of a branch should grow faster (to make a curve) are deactivated by direct light. 

So it's almost entirely dependent on the environment, to the point that if you're growing chillies in your window you can change their growth pattern by rotating the pot they're in.