Born to be what?

We're all mutants. Now, I de-glorified all Marvel comics heros/villains that posess some kind of extrordinary gift due to being a "mutant". Well, it's not that remarkable in the real world. You won't gain any cool abilities by exposing yourself to nuclear waste, except the ability to grow aggressive tumors in your body (otherwise, imagine what the flora and fauna around Chernobyl would look like at this time). Still, mutations occur all the time; in us and around us. By using the different mechanisms, that causes mutations, in the laboratory we may see glimpses of how nature could work.

In all animals, fungi and plants there are a group of genes called HOX-genes. These dictates the development from a single cell to the multicellular being that we become. Think of them as "on and off" buttons that can be connected in a huge network, giving thousands of possibilities for combining the signals. In simple words, the different combinations of HOX-genes determines the orientation of the future body, i.e. where the head should be, the legs and so on, as well as growing wings instead of an arm. This means that a small change in the combinations directs the developing body to a completely different structure than "normal". Usually, the consequences are so severe that the fetus is spontaniously aborted, but not all developmental disturbances are lethal.

The fruit fly Drosophila melanogaster have been studied to the core when it comes to the HOX-genes. By disturbing the pattern of expression of these genes, either with mutations or environmental factors (such as chemicals) a fly with antennae instead of eyes develops, or a fly with two thoraxes (see pictures further down for both). In mice you can disturb the development by turning "on" some of the HOX-genes way too much so that the new born mouse has no hair at all. So it's not as simple as just having a genome with all you need to build a being. The genome must be extremely fine-tuned in order to have all limbs in the correct places!

Sources:

http://www.musc.edu/mcbp/concentrations/geneticsdevelop.html