The son of Polish immigrants, my grandfather grew up on a polyglot street, and he began drinking with purpose in his teens. My grandmother stowed money in cans so that he wouldn’t spend it. He was a boxer. Tall and lithe, he fought amateur Golden Gloves bouts in his Chicagoan youth in the 1930s. (His brother Joe traveled to New York and sparred with heavyweight champ Jack Sharkey). He fought and he drank, inside the ring and outside, and it continued while he served the U.S. Army’s Engineer Corp.

He didn’t talk about WWII, but years later we learned of his station transfers through the jackbooted continent based on records of his stints in the brig. The poor aren’t born into this world; they come crashing into it. When he learned he was having a son, my grandfather and his brother-in-law celebrated with drinking, stumbling and shattering a store-front display window, landing in a heap of plate glass.

Years later, my father would scribble down notes in journals, detailing those past events, documenting hard lives that built the foundation for us. We had recovered my grandfather from those slippery streets, but his faulty memory couldn’t be rescued. In fact, his lack of recognition was jarring. When my father was preparing to remove his clothes from a dresser, he recalls, my grandfather protested, "You can't take those clothes, they belong to my son." Alzheimer’s "moments" drop like chasms.

And yet, this is not one elegiac story, it is many. Five million people in the United States have Alzheimer’s disease. This number will double in a decade. If any of us live to be 85, the chances of having the disease or some form of dementia is about one in three. The explanation for why some of us get it and some don’t is a largely unsolved genetic riddle. My grandfather eventually died from it. My grandmother is now 96-years-old, writes me cards with lacy cursive, and regularly beats me at Scrabble.

Recently, my father subscribed to a service that allowed us to mail in a cheek swab to learn about our genetic ancestry. I learned that I belonged to Haplogroup R—a type of ethnic branch on our genetic tree—that I am German and Polish (which I knew), and by a small fraction Ashkenazi Jewish (which I didn’t know), and I received a colorful map of my ancestors probable traipses through Europe. But though it was an option on the test, my father did not want to know about our risk for Alzheimer’s, it turns out. And for good reason—there is not a single meaningful drug to treat Alzheimer’s.

The role of genetics is far from clear for “late onset” Alzheimer’s. Consider that genes come in different versions, or alleles. The gene variant APOE4, which is a risk factor for the disease, differs from other versions by a single amino acid change. The mutation occurs in 14 percent of German people, for instance. How we live with APOE4 also matters. One in six boxers develops Alzheimer’s disease or Parkinson’s disease, known as “dementia pugilistica.” It can be useful to know if you carry APOE4. If you do, you shouldn’t overdo contact sports. Or start fistfights.