Heredity is broader than DNA transmission

Zimmer opens by dismantling the assumption that heredity simply means genes passed from parent to child. He shows that what we inherit also includes an entire ecosystem of microbes acquired during birth and infancy, cultural practices and family habits transmitted through upbringing, and even physical resources like inherited wealth or land that shape a life trajectory as powerfully as any gene.

He uses this expanded definition to challenge a common but misleading mental model: that a trait is either 'genetic' (fixed, inherited, biological) or 'environmental' (changeable, learned, cultural), as if these were cleanly separable categories. In reality most traits emerge from constant interplay between genetic predisposition and the specific environment a person happens to experience, making the either/or framing scientifically hollow.

This reframing has practical stakes: policies and personal decisions based on a narrow, genes-only view of heredity routinely misjudge how much of a trait is actually changeable. What passes from one generation to the next is a tangle of biology, microbes, and culture — not a single clean genetic thread.