|Fine. Then how are the parameters specified?|
I was helped to understand when it was first pointed out to me that, contrary to a lot of news articles, DNA is not analogous to a blueprint. It is somewhat more like a recipe.
A blueprint shows an image of the final result -- in fact that image is most of the blueprint.
A recipe may have a pretty picture of the result but it is irrelevant. You follow the steps and out comes (well, not so much for me ) something delicious.
The recipe doesn't say where each nut in the banana loaf goes the process distributes them.
The recipe doesn't say how high it will rise, the specifications of the recipe and the environment determine that within some range.
Changing something in the recipe may change the taste, height, density and lots more.