While cancer is common, purely hereditary cancer is relatively rare. About 5% to 10% of all cancers are linked to an inherited genetic mutation (like BRCA1 or BRCA2) that is passed down through families.

Decoding your family history means looking for specific patterns that suggest one of these genetic mutations might be present, rather than cancer just happening by chance or lifestyle factors.

Here is how genetic counselors and oncologists evaluate family history to determine hereditary risk.

The Red Flags of Hereditary Cancer

When mapping a family history, medical professionals look for specific warning signs. A single relative getting cancer at an older age is usually not a red flag. Instead, look for:

Early onset: Cancers diagnosed at an unusually young age (e.g., breast or colon cancer before age 50).

Multiple primary cancers: One person developing more than one original type of cancer (not cancer that has spread from one organ to another).

Related cancers on the same side: Multiple blood relatives on the same side of the family (either maternal or paternal) with related cancers. For example, breast and ovarian cancer are genetically linked, as are colon and uterine cancer.

Rare cancers: Conditions like male breast cancer, medullary thyroid cancer, or certain sarcomas are rare enough that they almost always prompt genetic testing.

Generational patterns: Cancer appearing in multiple generations (e.g., a grandfather, mother, and daughter).

How Genetic Counselors Map Your Risk

To decode this history, genetic counselors use a pedigree chart—a specialized medical family tree that tracks health conditions across generations.

How it works: In these charts, standard symbols (like squares for males and circles for females) are shaded to indicate someone who has developed a specific condition. Most hereditary cancer syndromes follow an autosomal dominant pattern, meaning you only need to inherit the mutated gene from one parent to have an increased risk.

Steps to Decode Your Own Family History

If you want to assess your own risk, gathering accurate data is the most crucial step.

Talk to blood relatives: Focus on your first-degree (parents, siblings, children) and second-degree relatives (grandparents, aunts, uncles, nieces, nephews). Do not ignore your father's side. People often mistakenly believe that "women's cancers" (like breast or ovarian) are only inherited from the mother's side. Men can carry and pass down these mutations too.

Get the exact diagnosis: "Stomach cancer" is often confused with colon cancer or liver cancer in family lore. Try to find out where the cancer originated, not just where it spread.

Find out the age of diagnosis: This is often more important than the age of death.

Note non-cancer findings: Some hereditary cancer syndromes cause benign tumors, unusual skin markings, or large head circumferences.