PZ Myers has a nice post on how the human (or any organism’s) genome does—and does not—work:
There is no blueprint, no map. That’s not how the system works. What you actually find in the genome are coding genes that produce proteins, coupled to regulatory elements that switch the coding genes off and on using a kind of sophisticated boolean logic. Each cell carries this complex collection of regulated genes independently and identically, but the boolean logic circuits produce different outputs varying with the inputs from the environment and the diverging histories of each cell. For instance, there is no code anywhere in the genome that commands the forelimbs to make five and only five digits: instead, a cascade of genes and cell movements produce a patterned tissue that in us contains sufficient mass and is of a size to generate five nuclei of condensing tissue that produce fingers.
It’s better to think in terms of cellular automata. The embryo is a pool of autonomous cellular robots that have general rules for how they should respond to environmental cues…and those cues tend to vary in predictable ways across the embryo, leading to a consistent cascade of action that produces a relatively consistent complex product, the multicellular organism.
The unfortunate consequence of those properties, though, is that you’ll never be able to look at a single gene from the genome and sort out what it does in the embryo. All the genes will be rather cryptic; you might be able to figure out that, for instance, the gene codes for an adhesion protein that makes the cell stick to a certain other class of cell, and that it’s switched on by gene products X and Y and turned off by gene product Z, but obviously you won’t be able to figure out its role until you figure out what activates genes X, Y, and Z, and whether the cell happens to be in a particular adhesive environment. And then when you look at X, Y, and Z, you discover that they have similar patterns of conditional logic in their expression.
In order to understand what a particular gene does, you have to understand what all the other genes do, as well as all the details of signaling and cell interactions that are going on, oh, and also, it’s entire developmental history, since epigenetic interactions can shape the future behavior of a cell lineage.
That’s only the beginning of a longer article, but well worth a gander.