Sara McGregor never worried about getting breast cancer. Neither did her two older sisters. After all, their mother and maternal grandmother never had the disease. So when McGregor's eldest sister was diagnosed at age 32, the news came as a shattering shock to all three siblings.

It was then that they learned their paternal grandmother (and her mother before her) had developed breast cancer at exactly the same age and died of it; until then, the sisters hadn't even realized that the breast cancer gene could be passed on from a father's side. Sadly, McGregor's oldest sister died at age 49; her middle sister was diagnosed at 47 but was successfully treated. McGregor opted for a prophylactic mastectomy and is cancer free.

McGregor has a lot of company in her ignorance of the key facts of her family's medical past. Too few of us make use of a simple, low-tech, easily accessed, and free tool that remains a powerful way to judge our risk of disease and then beat the odds: a complete family medical history.

Most of us are familiar with the doctor's office questionnaire that asks about illnesses we or any family members have had. These questionnaires—typically dashed off during a first visit—capture only information we recall offhand, and they might not be updated too often. A proper family medical history—what geneticists call a pedigree—is a far more complex and carefully constructed document that helps us and our doctors understand our risks of heritable or partially heritable diseases, including high blood pressure, Huntington's disease, type 2 diabetes, and many cancers.

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"You want to go back as far as your grandparents, then wide enough to include siblings, aunts, uncles, cousins, nieces, and nephews," says Joann Boughman, PhD, executive vice president of the American Society of Human Genetics. "For example, if your father has an inherited condition that causes repeat miscarriage, he may not be aware of it, because obviously he never got pregnant. That's why you need the broad context."

Constructing the best pedigree means collecting substantial information. In 2004, the US surgeon general declared Thanksgiving to be National Family History Day in the hope that as we gather, we'll also share information about who suffered from what and when. Also important:

The age at which a family member's health problems began.

This suggests what problems may be strongly hereditary. "A disease that occurs early—typically before age fifty—is more likely to have a strong genetic component," says Wendy Rubinstein, MD, PhD, medical director of the Center for Medical Genetics at NorthShore University HealthSystem in Evanston, IL.

Medical histories of relatives on both sides of the family.

"Many of us instinctively—but wrongly—believe that we share more genes with our same-sex parent," says Dr. Rubinstein. In fact, as the McGregor sisters learned, we inherit half our genes from each parent.

The races and countries of origin of family members.

Some problems are more common in certain ethnicities.

Discovering that several of your relatives had cancer doesn't necessarily mean it's time to worry, says Robert J. Gillies, PhD, vice chair of radiology research at the H. Lee Moffitt Cancer Center & Research Institute in Tampa. "But having multiple close relatives who had the same kind of cancer could suggest a condition that requires a consult with a risk assessment specialist," says Dr. Gillies.

The red flags are information to share--and possibly act on. Learning about your genetic risk factors allows you to take early steps to reduce the risk of developing those diseases, whether that means making lifestyle choices or getting appropriate medical screenings.

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EXPOSING YOUR ROOTS

Many families keep information about births and deaths in the family Bible, so start your search there. Some families don't like to talk about the real reason for a relative's death, and if it was long ago, doctors at the time might not have known. "Every family has skeletons," says Joann Boughman, PhD, executive vice president of the American Society of Human Genetics. "But I've never found one that doesn't have an aunt or a cousin who's the keeper of the secrets." These websites can help with a family health history:

SOLVE YOUR MEDICAL MYSTERIES

According to an old book her dad came across, Jessica Branch's great-great-great-great-great-grandfather died "of nightmare." Branch, a Prevention senior editor, had no idea why Amaziah Branch, a schoolteacher and preacher, and one of the first American published poets, would have had such a deadly dream. She felt that she'd sleep easier with a trustworthy translation of what she assumed was an antiquated medical term.

For guidance, we turned to Janet Golden, PhD, a professor of history at Rutgers University, Camden, who specializes in medical history. Her guess: "Amaziah died in his sleep, suddenly gasping from a stroke or arrhythmia."

If you're pursuing your pedigree, you'll find that ancestral causes of death can be vague. Dr. Golden says that diagnostic tools of yore were primitive, so many kinds of disease may simply have been listed as "sudden death." Here's what some of the most common terms may mean.

Old Term: Ague
Modern Translation: A bad cold or flu or any kind of fever

Old Term: Apoplexy
Modern Translation: Stroke

Old Term: Bad blood
Modern Translation: Syphilis. While you're unlikely to see this in the family Bible, you may find "locomotor ataxia" (motion disorder) in medical records—earlier generations didn't realize that muscle weakness was a symptom of syphilis.

Old Term: Childbed fever
Modern Translation: Puerperal sepsis, an infection introduced into the vaginal canal during birth

Old Term: Consumption
Modern Translation: Tuberculosis, usually—but this vague term can include other lung diseases, such as pleurisy, or infection

Old Term: Dropsy
Modern Translation: End-stage kidney disease

Old Term: Falling sickness
Modern Translation: Epilepsy

Old Term: Greensickness
Modern Translation: Anemia

Old Term: Lung fever
Modern Translation: Pneumonia

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