What Your Family Tree Might Be Telling You About Cancer Risk

By Amanda Shrewsbury, MS, CGC

You've probably heard the phrase "cancer runs in my family" at kitchen tables, in doctor's offices, and in quiet conversations between siblings. It's a phrase that carries weight, but it doesn't always carry clarity. Whether you're looking at your family tree and wondering if there's a pattern, or you've recently been diagnosed with cancer and your doctor suggested genetic testing, understanding the connection between family history and cancer risk is a powerful first step.

Here's what matters: most cancer is not inherited. Roughly 90–95% of cancers develop from a combination of aging, environmental exposures, and chance. But for a meaningful minority, about 5–10%, the underlying cause is a genetic variant (a change in DNA that increases cancer risk) passed from parent to child, generation after generation (National Cancer Institute, 2024). Some researchers estimate the true proportion may be even higher when familial clustering is included (Manchanda et al., 2020).

The challenge is that hereditary cancer can look like bad luck when you don't know the patterns. A grandmother with breast cancer at 45. An uncle with colon cancer at 48. A cousin diagnosed with ovarian cancer in her 30s. Taken individually, each might seem unrelated. Mapped together on a family tree, they can tell a very different story.

Why Does Understanding Family History and Cancer Risk Matter Now?

Because the vast majority of people who qualify for hereditary cancer genetic testing never receive it. Despite decades of advances in genetic testing, utilization remains startlingly low. A landmark study of over 1.3 million patients diagnosed with cancer in California and Georgia found that only 6.8% received germline genetic testing (a blood test that checks for inherited DNA changes) within two years of their diagnosis, even when national guidelines recommended it (Kurian et al., 2023). And cascade testing, the process of testing at-risk relatives after a disease-causing variant is found in the family, reaches only about one-third of eligible individuals (National Cancer Institute, 2023).

This isn't a failure of science. The tests exist. The clinical guidelines from the National Comprehensive Cancer Network (NCCN) are well-established and updated annually. The gap is between what's available and what's actually happening. That gap costs lives.

The good news? You don't need a medical degree to start closing it. You just need to know what to look for.

What Family History Patterns Suggest Hereditary Cancer?

The following five patterns are drawn from NCCN Genetic/Familial High-Risk Assessment guidelines (NCCN, 2025). If any of these show up in your family, it doesn't guarantee you carry an inherited variant. But it does mean that a conversation with a board-certified genetic counselor could be one of the most valuable healthcare decisions you make.

1. Cancer Diagnosed at an Unusually Young Age

When cancer appears decades earlier than expected, it often signals something beyond chance. Think breast cancer before 50, colorectal cancer before 50, or endometrial cancer in a premenopausal woman. The specific age that raises concern varies by cancer type, but the principle is consistent: younger onset suggests a stronger genetic contribution.

A 42-year-old with breast cancer has a fundamentally different risk profile than a 72-year-old with the same diagnosis.

What you can do:

Write down each affected relative's cancer type and their age at diagnosis
Focus especially on first-degree relatives (parents, siblings, children) diagnosed before 50
Don't assume that "young" means the same threshold for every cancer type. A genetic counselor can help you interpret what's significant for your family

2. Multiple Relatives on the Same Side With Cancer

Hereditary syndromes pass from parent to child, so cancer tends to cluster on one side of the family. Two or more close relatives on the same side, especially with related cancer types like breast and ovarian (associated with BRCA1/2, genes involved in DNA repair) or colorectal and endometrial (associated with Lynch syndrome, a condition caused by variants in DNA mismatch repair genes), strongly suggests a shared inherited cause.

One common misconception worth correcting: cancer on your father's side matters just as much for breast and ovarian cancer risk. These genetic variants don't skip a generation or affect only one sex. They can be passed equally by mothers and fathers.

What you can do:

Map your family history separately for each side (maternal and paternal)
Include aunts, uncles, and grandparents, not just parents and siblings
Pay attention to which cancers appear together, not just the total count

3. Rare or Unusual Cancer Types

Certain uncommon cancers are hallmarks of hereditary syndromes. Even a single case can be clinically significant. These include:

Ovarian, fallopian tube, or primary peritoneal cancer
Male breast cancer
Triple-negative breast cancer (a specific subtype defined by what the tumor lacks rather than what it has) diagnosed at any age
Pancreatic cancer
Metastatic or high-grade prostate cancer
Rare tumors like adrenocortical carcinoma (associated with Li-Fraumeni syndrome, a condition caused by variants in the TP53 gene)

What you can do:

Document any of these in your family, even if it's the only case
Ask about specific subtypes when possible. Triple-negative breast cancer has a meaningfully different genetic profile than hormone-receptor-positive breast cancer
One case of ovarian cancer or male breast cancer is enough to warrant a genetic counseling conversation

4. One Person Diagnosed With Multiple Primary Cancers

When someone develops two or more separate primary cancers (meaning two independent cancers, not a recurrence or spread of the original), it may point to an inherited predisposition. Classic examples include:

Breast and ovarian cancer in the same person
Colorectal cancer followed by endometrial cancer (a Lynch syndrome pattern)
Bilateral breast cancer (cancer in both breasts), especially if the first was diagnosed young

What you can do:

Note whether any relative had more than one cancer type, and at what ages
The distinction between a new primary cancer and a recurrence or metastasis (cancer that spread from the original site) matters. The genetic implications are different
Multiple primary cancers in one person is one of the strongest single indicators of a hereditary syndrome

5. A Known Genetic Variant Already in Your Family

If a relative has tested positive for a pathogenic variant (a disease-causing DNA change, sometimes called a "mutation") in a gene like BRCA1, BRCA2, PALB2, or one of the Lynch syndrome mismatch repair genes (MLH1, MSH2, MSH6, PMS2, EPCAM), close relatives may carry it too.

Cascade testing looks specifically for the known family variant. It gives a definitive yes-or-no answer, is typically covered by insurance, and is widely regarded as one of the highest-value, lowest-cost actions in all of preventive medicine.

What you can do:

Ask family members whether anyone has had genetic testing and would be willing to share results
Even if the tested relative has passed away, their results can still guide the entire family
Don't let the conversation end with one person's diagnosis

Will Genetic Testing Affect My Insurance?

A concern we hear often: "If I get tested and something is found, will it affect my insurance?"

The Genetic Information Nondiscrimination Act (GINA) is a federal law that prohibits health insurers and employers from discriminating based on genetic test results. It's important to know that GINA does not extend to life, disability, or long-term care insurance, though some states (including Colorado) have enacted additional protections. A genetic counselor can walk you through the specifics for your situation.

What to Do Next

If any of these patterns showed up while you were reading, the single most important next step is meeting with a board-certified genetic counselor: a specialist trained to evaluate family history, assess risk, determine whether testing is appropriate, and guide you through results and next steps. (Not sure what to expect? Read our walkthrough: What Actually Happens During a Genetic Counseling Appointment. Already have results? See What Your Genetic Test Results Actually Mean.)

Before your appointment, try to gather:

Which relatives had cancer and on which side of the family
Cancer type and approximate age at diagnosis for each
Whether anyone had more than one primary cancer
Any prior genetic testing results in the family
Your ethnic background (some variants are more common in certain populations)

Don't worry if your information is incomplete. A counselor can work with whatever you know.

Ready to Take the Next Step?

ClearPath Genetics offers virtual genetic counseling with board-certified counselors who specialize exclusively in hereditary cancer risk. You can meet with an expert from the comfort of your home.

References

Kurian, A. W., Abrahamse, P., Furgal, A., Ward, K. C., Hamilton, A. S., Hodan, R., Tocco, R., Liu, L., Berek, J. S., Hoang, L., Yussuf, A., Susswein, L., Esplin, E. D., Slavin, T. P., Gomez, S. L., Hofer, T. P., & Katz, S. J. (2023). Germline genetic testing after cancer diagnosis. JAMA, 330(1), 43–51.
National Cancer Institute. (2024). The genetics of cancer. U.S. Department of Health and Human Services.
National Cancer Institute. (2023, August 1). Few people with cancer undergo germline testing. Cancer Currents Blog.
Manchanda, R., Patel, S., Gorber, B. S., et al. (2020). Cost-effectiveness of population-based BRCA1, BRCA2, RAD51C, RAD51D, BRIP1, PALB2 mutation testing in unselected general population women. Journal of the National Cancer Institute, 112(7), 714–724.
National Comprehensive Cancer Network. (2025). Genetic/familial high-risk assessment: Breast, ovarian, pancreatic, and prostate (Version 2.2025).
National Comprehensive Cancer Network. (2025). Genetic/familial high-risk assessment: Colorectal, endometrial, and gastric (Version 1.2025).
Genetic Information Nondiscrimination Act of 2008, Pub. L. No. 110-233, 122 Stat. 881.