“My paternal grandfather had breast cancer.” That always makes whoever is charting my medical history look up.

“He had a radical mastectomy in the 1970s. And his sister had it, too — she died young. And one of his nieces. And his daughter — my aunt.”

At age 37, I have just been diagnosed with breast cancer, and the genetic counselor is furiously sketching out my family tree on a sheet of paper. There are squares and circles, the cancer victims marked with X’s. Lots of X’s.

On my mom’s side: cancer in both her parents, although not breast. Melanoma in her sister, my aunt. And less than six months after this conversation, my mom herself will be dead from a blood cancer called multiple myeloma.

As the genetic counselor is drawing the diagrams, I am remembering a similar one from seventh-grade science class, the Punnett square: almost fortuneteller-like, better than a Ouija board and those folded-up cootie-catchers — when the grown-up self is almost equally conceivable and impossible. Pick any boy in the class, and you could predict the likelihood that you and he would have kids with brown eyes or hair on their toes and fingers. Or — as the genetic counselor’s diagram seems to suggest — cancer.

Nina Riggs was diagnosed with breast cancer when she was 37. Cancer is common on both sides of her family. (Courtesy of Nina Riggs)

According to the Punnett square, two kids at my table, Mike Henninger and Christina Stapleton, had a 100 percent chance of having a blue-eyed baby. This thrilled seventh-grade me: Something about the future was settled, then. A certainty — if Christina and Mike fall in love. And want a child. And Christina is able to get pregnant. And the baby arrives safely into the world.

Another version of certainty: In the months leading up to her death, talking about funeral plans brought my mom comfort. She insisted on cremation and wanted her ashes scattered at our family place on Cape Cod, a rustic house that was built by my dad’s grandfather in the 1930s. Even though my mom started going there only after she married my dad, it’s an obvious choice. The place — isolated, dramatic, exposed — has an undeniable otherworldliness to it. My cousin’s ashes are buried up there. My uncle’s initials were carved into a boulder there after the small plane he was piloting went down about 20 miles offshore.

On my dad’s side: His older sister has the breast cancer mutation BRCA2. She was the first of us to be tested, after her diagnosis in the 1990s. Her daughter, who has not had cancer, also has the mutation. And so does at least one of my dad’s three living brothers.

But it turns out I do not have it. I have just been diagnosed with an aggressive breast cancer, but I do not have the breast cancer mutation.

“I’m going to send you a study I found,” the genetic counselor tells me. “You might be interested in the findings, given your situation.”

Researchers have discovered that in families where there is an identified breast-cancer-gene mutation such as BRCA1 or BRCA2, even family members without the mutation are at a greater risk for developing the disease.

“All this likely means is that there are some genes we have not successfully mapped yet,” the counselor says. “We are seeing part of the picture, but not all of it.”

We are certain only that there is so much of which we are not certain. There is no Punnett square for life.

As far as we know now, genetics accounts for only about 11 percent of all breast cancers. Which leaves 89 percent hurtling randomly toward us through outer space.

My grandfather, the one with breast cancer, died when I was 7, two years after my grandmother. Cancer, both of them — his maybe metastasized from the breast, maybe something else. We can’t be sure — it was the early 1980s.

“Did you ever see his scars,” asks one of my uncles after my diagnosis, “from the mastectomy?”

Once I did, although at the time I thought they were from a war. It was summer. I was 5 or 6 years old and we were down on the rocky beach below the Cape Cod house, where my grandmother’s horse Sachem had caught a leg between two large rocks, snapped it with the force of his own heaving and had to be shot. The horse’s body was too immense to move, and everyone was sweating from the work of covering him with a mound of rocks piled taller than me.

My grandfather’s body was lean and muscled and rigid — the familiar Riggs physique — but his bare chest was another planet: distorted, twisted with scar tissue, hollowed out to the rib cage like a wooden-hulled skiff.

Grown-ups are full of surprises, I remember thinking. Who could ever possibly imagine what it is to be one?

Sitting in the genetic counselor’s office a year and a half ago, I didn’t know it yet, but my own scar wouldn’t be quite so intense. It is a stretched S-shape snaking about eight inches from my sternum to just under my armpit. My husband sees a sideways Superman-type S. I see a lazy question mark with the much smaller scar from the surgical drain as the dot.

Years after the day I saw my grandfather’s scar, farther down the beach where the bluff curls to a weedy cove, some of Sachem’s bones eventually returned to us — bleached, worn and so massive at first I imagined they belonged to a prehistoric beast. Now one is kept on the table near the mantel, next to the angry jaw of a bluefish, the slough of a king snake, a brittle helix of thousands of conch eggs and two wooden plaques carved with my grandparents’ dates.

Some things are meant to return to us again and again.

Six months after my mastectomy, the cancer returned, spreading to my spine and breaking one of the vertebrae in my back. I went to see an acupuncturist to help with the pain. She has a theory of “familial patterns” of behavior: emotional, nutritional, regional, etc. I twitch a little at the lack of hard science behind this characterization, but there is something about it that appeals to me more than genetics: It feels mutable, less fated. And I can almost map it.

“It is significant that your cancer attacked the same vertebra as your mother’s did,” she says one day when I tell her that myeloma broke my mother’s back in the same place. “That vertebra is linked to the liver, which is the seat of anger.” Sounds like a stretch, I know, but throughout her life my mom struggled with anger issues related to her childhood and the family she felt she never belonged to. I had issues with my mom’s anger issues: I was scared of them, yet I married a man with similar ones. Now we both struggle with our older son’s anger problems. It’s enough to break your back, really, these patterns.

Now, a year after the mastectomy and 10 months after my mom’s death, the whole area around my scar is still numb, so tracing it with my fingers is the disorienting gap between the expected and the perceived. The terrain is treacherous in some stretches, flat in others, with a fine ridge that slopes through the shallow crater of my right chest. It is not lovely, exactly, but it is — to my fingers — unforgettable. And, I’ve realized, it’s familiar.

It’s not a Superman S or a question mark. It’s a path, the one that everyone in my family knows pretty well, the one that starts by the boathouse, above the rocks down by the weedy cove, then weaves its way past the cat briar and beach roses and starts to climb up the hill through grass thick with berries and ticks and poison ivy, curving gently this way and that up the bluff to the house, where I can picture just now my mom gliding out of sight, stepping inside from the porch through the sliding door.

About two weeks after she died, we scattered her ashes into the tall grasses. It was Labor Day, the breeze sharp and clarifying, the sun still warm, the ocean darkening to a rich navy. Maybe 50 members of my dad’s family — the family that had become my mom’s family — gathered at her favorite spot by the flagpole, the spot where I like to picture her in the Adirondack chairs, chatting with her sisters-in-law and holding my infant son as he dozes to the lap of the waves and the whine of inboard motors going by on the water.

That day, many of us took handfuls of her ashes and offered them up to the breeze. The wind was such that for a moment each handful hung in the air like a beautiful specter contemplating our group — nearly returning to us, then spirited away, sometimes almost a recognizable shape, sometimes something entirely unfamiliar.

My two sons, 6 and 9 now, know my cancer will never go away — but, not unlike the scientists and doctors, they hardly know what that means. Loss, like genetics and adulthood, is full of mystery.

“Will I get a scar like that when I’m older?” asks the younger one, watching me dress one morning.

“I really hope not,” I say.

“Well, I really hope I do,” he replies. “That way I’ll be a little more like you.”