Dude, cell expression is fucking crazy. You start off with these undifferentiated stem cells and what features they develop depends on where they end up in your body. How? Cell signaling. Your cells can chitchat with each other using chemicals and cause large-scale change, no central nervous system needed. Ever wonder how zebras get their stripes, or why you have 5 fingers instead of 10? Same thing. Fun fact: Alan Turing studied the mathematics behind that stuff back in the 50s. What's even weirder is that a decent number of things about you are determined not by your own DNA, but by the DNA of the several pounds of bacteria that live in and on you. There's definitely a grain of truth to the idea that eating dirt is good for kids. Did you know you can extract DNA from an onion using mostly household kitchen equipment? Seems like a decent starting point for some DNA rope experiments...