Q: I thought I was born with my genes (some from Mom, some from Dad), and they made me who I am. Now I hear that how you live day-to-day turns genes on and off. Does that change who I am, basically? – Mitch D., Mystic, Conn.

A: You were born with a set of genes. The DNA-containing chromosomes in those genes determine your gender, your eye color, even your sensitivity to sound. But not all genes are active all the time. And about 80 percent of your genetic material is made up not of genes but of gene switches (which we are just beginning to understand). These switches cause changes in the way a gene works; this is called gene expression — a smile or a grimace, if you want to think of it that way. Switches can be prodded by things like toxins, stress, behavior and diet into expressing themselves as they were intended to — or not — without altering your DNA. So you stay the same person but one who, unfortunately, develops cancer, muscular dystrophy or mental illness, for example.