During the development of a complex organism, a fertilized ovum, or zygote, divides in two, then again into twice as many cells and eventually into all the cells that compose the organism's body. As the cells proliferate, they differentiate in form and function into the various cell types of that particular kind of body. This differentiation into skin, stomach, nerve, and other cell types occurs even though the cells of a developing body all share a common genotype, that of the original zygote. The paradox of one genotype yielding many cellular phenotypes has been resolved, in a general sense, through the mechanisms of epigenetics. A relatively new branch of molecular biology, epigenetics addresses issues related to gene regulation and gene regulatory networks. The new discipline aims to explain how, during development, genes get turned on and off and when (as in larval or adult forms of organisms) and where (as in spleen or kidney) they do.
The new discipline is an upstart. Epigenetics would seem to demote DNA from being the cell's chief executive to its merely utilitarian, dumb server. DNA includes an archive of messenger-RNA templates (and the messenger RNA molecules transcribed from the templates still pass through an editing suite before being escorted to the ribosome, where they get translated into proteins). The molecular machinery of epigenetics, through normal chemical bonding, excites or inhibits DNA "expression" or "action." The countless combinations of sections of DNA that can be expressed and repressed here and there in sequence or in tandem produce multiform cellular phenotypes from the highly conserved DNA of the original zygote.
From a complex database a skilled operator can extract many kinds of reports, by slicing the data this way then that. DNA is such a complex database, responding to many and diverse calls for data. The creatures of the Earth are reports summoned from DNA, not expressions of any executive talent that resides in the DNA. This is the new view of things from the world of epigenetics.