Knowing your family history could save your life. (Andrey Popov/iStock)

In 2014, Diana visited our genetics clinic in Texas. She was only in her early 40s, but her mother had died of breast cancer at 42, and two of her aunts had received the same diagnosis at young ages. As we tested her for a panel of breast cancer gene mutations, she joked about her Irish heritage, of which she was clearly proud. But we discovered that Diana carried an Ashkenazi Jewish BRCA1 mutation that predisposed her to breast and ovarian cancer. During World War II, it turned out, her French Jewish family converted to Catholicism and made Ireland their home. Nobody had told her.

The secrecy was understandable. But as an oncologist, I know it can carry major risks. One of the most important things I need to learn about my patients is their family medical history. Many illnesses — from diabetes and asthma to heart disease and dementia — are at least partly hereditary. One recent study found that 33 percent of cancer diagnoses can be explained by genes. Alcoholism is as much as 60 percent genetic; schizophrenia occurs in less than 1 percent of the general population but in 10 percent of people who have a sibling or parent with the disorder. “What happens in your family is an approximation of what happens to you,” geneticist Joe Nadeau told New York magazine. “The conventional family history is still the best predictor of disease risk.”

For many patients, sharing this information could save their lives. If a patient’s family is peppered with alcoholics, I might advise him or her to avoid liquor. If a patient’s relatives have suffered from colon cancer, I’d know to look for gene mutations and prescribe frequent colonoscopies.

But few of us have an accurate understanding of what came before us, health-wise. Only one-third of Americans have ever tried to collect their family medical histories from relatives. And even when they do, they can run into major problems. You probably know less than you think about your family’s predispositions.

Part of that is the fault of the medical profession. Doctors spend very little time — just 15 minutes, in a lot of cases — with patients. We simply can’t afford to construct a family tree. One study found that physicians devote just three minutes to asking family-history questions during a patient’s first visit.

Even when doctors ask, they may not get the right answers. Many patients don’t really know their families’ health histories. In some instances, relatives might never have revealed that they were sick, particularly if they struggled with a disease that carries a stigma. According to one study, 58 percent of psychiatrists said they wouldn’t tell family and friends if they suffered from a mental illness. Cancer advocate Doug Ulman wrote that he’s known many people to keep their illness under wraps because of “fear, ostracism and shame.” I’ve seen this in my family, too. My 94-year-old mother loves to talk about her father’s daily visits to speakeasies and his 1931 imprisonment for drunken driving. But she would never describe him as an alcoholic.

Other times, patients are aware of only the outlines of their families’ medical histories, or they interpret them incorrectly. One person visiting our genetics clinic denied that he had a family history of cancer because, he said, nobody had died. Another suggested that her sister’s breast cancer at age 38 was due to her divorce. One even told me her father had only “a touch of melanoma.”

And when we can look back at family records, we might not understand what we’re reading. It wasn’t so long ago that tuberculosis was known as “consumption” and epilepsy was “falling sickness.” Strokes were “apoplexy,” and “bad blood” was code for syphilis.

Studies show just how widespread this problem is. In one study, researchers asked more than 1,000 people about cancers in their relatives and then checked those responses against information in registries, Medicare databases, death certificates and other health records. Three-quarters of the histories that patients reported had errors.

Even with official health documents or family stories, we need to remain skeptical. Mistakes in records happen. And memory is not a factual accounting of past events but a construction of what we think happened. Lung cancer may have, in fact, been colon cancer; ovarian cancer perhaps was cervical cancer. This matters because different types of cancers put patients at risk for different kinds of diseases.

I had a colon cancer patient who was adopted at birth. Before starting a family, he wanted to learn more about his biological parents and their health histories. As a psychiatrist, he was concerned about mental disorders that have a hereditary component, such as schizophrenia. After much work with an adoption disclosure agency, he met his birth parents. He learned that they had been unmarried when he was born.

He asked his biological parents about mental illnesses. None, he was assured. He didn’t think to ask about cancer.

He and his wife had two children. Then, at 42, he was diagnosed with colon cancer. He contacted his biological mother to let her know. She wasn’t surprised. You should know, she told him, that others in the family had suffered from the same disease.

When he pressed, he found out that “others” also included secret siblings. His biological parents had told him only about siblings who were born after they married. They hadn’t shared the medical histories of the other children who had also been given up for adoption.

Eventually, he met the rest of the family. One of his siblings was about to cancel her regular colonoscopy when she learned about the colon cancer in her newly discovered brother. She kept the date, and the gastroenterologist discovered polyps, a precursor to colon cancer. The polyps were removed, and her life was most likely saved.

It’s rarely easy to learn about your family’s health history, but it’s worth doing your research. (The Centers for Disease Control and Prevention and the National Institutes of Health have online guides to help people get started.) Knowing what you know — and don’t — about your family’s medical history is one of the most important tools there is to save lives in predictable and, more often than not, unpredictable ways.

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