Originally posted by Darat
Here's a The StraightDope article about it:
http://www.straightdope.com/classics/a980821.html
Cecil uses an apt metaphor here:
"much of our physical makeup seems to be improvised [ ] not by some master jazz musician, but by a collection of stupid molecules."
Think of the DNA as a musical score which acts as guideline, and the cells as individual musicians. Never mind jazz, the same piece played by two different
classical orchestras -- though comprised of the same instruments -- may have quite different sounds, and even the very same musicians may, through slight differences in timing or emphasis, interpret the same piece differently on separate occasions. If we measured those nuances closely enough, we would find that the same piece is
never played
exactly the same way twice.
There is a reciprocal relationship between the DNA and the individual cell in which it resides. The chemical state of the cell plays an important role in gene expression, one which might be considered analogous to the subtle differences in timing or emphasis --
improvisation -- in the orchestra metaphor above. This is especially true during embryonic development, when individual cells are 'deciding' what kind of cells to be.
Sufficiently close examination of
any structure two organisms have in common would reveal that the corresponding individual cells are
never in exactly the same arrangements -- not even in clones. Fingerprints are just a particularly convenient place to make that comparison.