One of my earliest, and worst, memories involves poking my head into the bedroom where my grandfather, laid low by Parkinson’s disease, lived out his days. A white-haired version of my father confronted me, propped in a bed behind metal railings. I remember lots of white—the wallpaper, the carpet, his smock. He moaned at me, and syrupy saliva trembled on his chin. I was only five. I’m still ashamed I ran.

Twenty-some years later, I found myself filling a plastic tube with my own saliva, to submit for genetic testing. I had no reason to think of Grandpa, and filled the genetic spittoon in a frivolous state of mind. I merely wanted a surprise (maybe a Genghis Khan gene) to spice up my new book, The Violinist’s Thumb. Weeks later, I logged on to view my test results online—and finally read...

I’m not alone. Even scientists fear their genes on some level. Genes deal in probabilities, not certainties, but it’s near impossible to accept that having a DNA signature for some disease doesn’t doom you to develop the disease itself. Nobel laureate James Watson—haunted by memories of his Alzheimer’s-ridden grandmother—refused to find out which versions he carried of a certain Alzheimer’s-linked gene (apoE) when his genome was sequenced.

While writing The Violinist’s Thumb, however, I learned that iconoclast geneticist Craig Venter had published his entire genome, uncensored, consequences be damned. I admired his aplomb in facing down his DNA, and with every day that passed, my conscience nagged me to do the same with Parkinson’s. Eventually I logged on again, and clicked to break the electronic seal.

Relief flooded through me: I had no increased genetic risk for Parkinson’s. I whooped. I rejoiced—but should I have? Genes deal in probabilities, not certainties. That was my mantra before I peeked, to convince myself that even the riskiest DNA wouldn’t inevitably ravage my brain. But when things looked less grim, I happily dispensed with uncertainty, happily ignored the fact that lower-risk DNA didn’t mean I’d inevitably escaped. Parkinson’s might still come—yet my relief was genuine. It’s the paradox of personal genetics.

Over the next months I shooed away this inconvenient cognitive dissonance and finished The Violinist’s Thumb. The day I dotted the last i, the testing company e-mailed me updated results. Fortified and foolhardy, I clicked right through.

As I scrolled down, my eyes lit on some large green letters: “Slightly higher odds of developing Parkinson’s disease.” Higher? A new study had scrutinized a different spot in the genome, on chromosome four. Most Caucasians have “CT” or “TT” there. I had “CC,” which meant higher odds.

I’d been double-crossed! To expect and get a genetic condemnation is one thing. But to expect it, get pardoned, and find myself condemned again? Infinitely more torture.

Somehow, though, receiving this genetic sentence didn’t tighten my throat. I felt no panic, no fight-or-flight jolt. I wasn’t pumped up, but I felt relatively tranquil.

So what happened between the first and second revelation, between the setup and the would-be knockdown? I guess I got an education. I knew now that for a complex disease like Parkinson’s, any one gene contributed little to my risk. Plus, “higher odds” meant just twenty percent higher, and that for a disease that affects just 1.6 percent of men. Perhaps in the generational shuffling of genes, the dangerous bits from Grandpa had been dealt out anyway. There’s no certainty with genes, good or bad. But there’s no reason for the little boy in me to keep fleeing.

Sam Kean is the author of  The Disappearing Spoon, a New York Times bestseller. His work has appeared in the New York Times Magazine, Slate, and Mental Floss, and has been featured on “All Things Considered” and “Radiolab.” Kean is a Washington, DC-based writer who was a runner-up for 2009’s National Association of Science Writers’ Evert Clark/Seth Payne Award for best science writer under the age of 30. Read an excerpt of  The Violinist’s Thumb.

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