One of the things that I really reset my thinking about during my online biology course with Eric Lander was the misleading notion of genetics as being about traits or blueprints.
What I somehow missed in my naïve, school-boy understanding before was that genes control the operation of cells (not organisms directly) and throughout their life (not just during development).
The genetic code is not a collection of traits the organism will have, nor a blueprint for building an organism. It is the ongoing instruction manual for individual cells.
This may seem obvious to anyone with even a modicum of biological knowledge but Gary Marcus's book The Birth of the Mind makes clear how widespread the misunderstanding is/was (even amongst biologists earlier in the 20th century).
And people still talk about things like the "gene for blue eyes" without, in most cases I suspect, realizing how much of a misleading description this is of what's going on.
I don't think I grasped gene expression even at a basic level until I got rid of my bad thinking.