Complex organisms cannot be construed as the sum of their genes, nor do genes alone build particular items of anatomy or behavior by themselves. Most genes influence several aspects of anatomy and behavior—as they operate through complex interactions with other genes and their products, and with environmental factors both within and outside the developing organism. We fall into a deep error, not just a harmful oversimplification, when we speak of genes for particular items of anatomy or behavior.
"The Without and Within of Smart Mice", p. 234 (originally appeared in Time, 1999-09-13) - I Have Landed (2002)