We know that the DNA contains all the instructions for building a body – but not in an architectural sense. There’s no picture, no blueprint. For me, it’s a bit more like turtle graphics (http://sonic.net/~nbs/webturtle/, http://fmslogo.sourceforge.net/). Turtle graphics allows you to move, turn, pen down/up, maybe even change colour. Sequentially following the instructions allows you to draw complex pictures. DNA has a greater range of instructions, and many operations are happening in parallel, but essentially it’s the same deal – no master plan, merely morphology emerging from the unfolding sequence of algorithms.
The DNA instructions, read at some higher level than mere codons or genes, might sound more like “Build this thing, then that. When you get here, turn, when you get there, stop. If it’s hot, build this, and stop building that. Always make sure you have plenty of these.”
Mutations in genes are like errors in a line of the program. Most are nonsense and will cause it to crash. A few will change the morphology of the picture, fewer still will make it better.