Why Two People With the Same Gene Variant Can Have Very Different Cancer Risks

By Amanda Shrewsbury

Two people can carry the exact same variant in the exact same gene and live very different lives, one facing cancer in her forties and the other reaching eighty without ever being diagnosed, and understanding why that happens is one of the most useful things a person with a hereditary cancer variant can learn.

This is one of the most misunderstood parts of hereditary cancer genetics. A positive genetic test result is often heard as a verdict, a fixed and certain future. It is not. It is an elevated risk, and elevated risk is a very different thing from certainty.

Being clear about this matters in both directions. Understating the risk would be its own kind of harm, because these risks are real and they are worth acting on. But treating a risk percentage as a prediction is not accurate either, and it can make people feel that the outcome is already decided when it is not. This post explains the concept behind that distinction, why the same variant behaves differently in different people, and what that means for how you should think about your own numbers.

Does everyone with a hereditary cancer variant get cancer?

No. Some people who carry a pathogenic variant (a DNA change that increases cancer risk) never develop cancer at all.

This is well established across hereditary cancer syndromes, not a rare exception or a hopeful footnote. Guidance from the American College of Obstetricians and Gynecologists notes that hereditary breast and ovarian cancer syndrome, along with many other hereditary cancer syndromes, shows this pattern (ACOG, 2019). Patient-facing gene summaries from FORCE say the same thing plainly on individual gene pages: the lifetime cancer risk associated with a variant may be high, and still, not everyone who carries that variant develops cancer.

This also explains something families often notice and misinterpret. A parent can carry a variant, never develop cancer, and pass that variant to a child who does. It can look as though a condition skipped a generation. Genes do not skip generations, but cancer can appear to, because carrying a variant and developing cancer are not the same event.

What is penetrance?

Penetrance is the term for this. It refers to the proportion of people carrying a particular variant who actually develop the associated condition.

When penetrance is described as incomplete, it means some people with the variant do not develop the condition. Most hereditary cancer variants show incomplete penetrance. This is the single concept that explains why a genetic result gives you a probability rather than an outcome, and why your genetic counselor talks in ranges rather than handing you one number.

Do all hereditary cancer genes carry the same level of risk?

Not at all, and this is the first reason two people''s risks can differ. Penetrance varies enormously from gene to gene.

Some genes are described as high-penetrance genes, meaning a single variant in that gene confers a substantially elevated lifetime cancer risk. BRCA1 and BRCA2 are the most familiar examples. Other genes fall into a moderate-penetrance category, where a variant meaningfully raises risk without reaching the same level. Genes like ATM and CHEK2 are often discussed in this group.

The practical consequence is that a positive result is not one thing. A person with a BRCA1 variant and a person with a CHEK2 variant have both tested positive, and their screening recommendations and risk conversations can look quite different. Our post on the hereditary cancer genes beyond BRCA1, BRCA2, and Lynch syndrome walks through these genes individually and is the better place to look for gene-by-gene detail.

Why do two people with the same variant have different risks?

Here is the harder and more interesting question. Set aside differences between genes. Take two people with a variant in the same gene, sometimes even the identical variant. Their risks can still differ, and often do.

The reason is that a variant does not act alone. It sits inside a person, and that person has their own family history, their own genetic background, their own reproductive and hormonal history, and their own exposures across a lifetime. Researchers reviewing hereditary breast cancer describe exactly this list of influences on penetrance: age, sex, family history, polygenic background, lifestyle, reproductive and hormonal factors, and environment. A variant sets a baseline. Everything else shifts where a person sits relative to it.

The sections below cover the modifiers that matter most in a clinical conversation.

How does family history change risk for someone who already has a variant?

This is the best-documented modifier, and it is the one your genetic counselor is most likely to bring up.

A large prospective study of BRCA1 and BRCA2 carriers found that breast cancer risk increased as the number of first- and second-degree relatives diagnosed with breast cancer increased (Kuchenbaecker et al., 2017). In other words, among people who all carry a BRCA1 or BRCA2 variant, those with more affected close relatives faced higher risk than those with fewer.

This has a real consequence for how results are interpreted. Consider two people who both carry the same BRCA2 variant. One comes from a family with several relatives diagnosed with breast cancer across multiple generations. The other learned about her variant incidentally, through panel testing done for an unrelated reason, with no known family history of cancer at all. Both results are positive, both warrant careful management, and their risk estimates may not be identical. Family history is not just how we decide who to test. It stays relevant after the result comes back.

Do other genetic factors play a role?

Yes, and this is an active area of research rather than a settled one.

The rest of a person''s genetic background appears to modify the risk associated with a hereditary cancer variant. The National Cancer Institute notes that the observed variation in penetrance is what led researchers to propose that other genetic and environmental factors are at work (NCI, 2025). Studies have since identified common genetic variation that appears to shift breast cancer risk up or down among people who carry BRCA1 or BRCA2 variants.

For most people this is context rather than something actionable today. It is worth knowing because it explains part of the mystery. Two people with the same variant are not genetically identical everywhere else, and those differences appear to matter.

Do lifestyle and other factors matter?

Yes. Reproductive history, hormonal factors, and lifestyle and environmental exposures are among the influences researchers include when describing what affects penetrance, and they are worth paying attention to. Our post on everyday choices and cancer risk covers this territory in more depth.

To be clear, no one brings cancer on themselves. These factors shift risk; they do not decide outcomes, and plenty of people who do everything right still develop cancer.

Why are cancer risk estimates given as a range instead of one number?

Because every source of variation above ends up inside the estimate.

A published penetrance figure is a population statistic. It describes what happened across a group of people who carry variants in a given gene, and the people in that group differed from one another in all the ways just described. Historical estimates varied widely depending on how a study recruited its participants, which is part of why older and newer figures do not always match. The 2017 prospective cohort study was designed specifically to produce more reliable estimates than the earlier retrospective work (Kuchenbaecker et al., 2017).

So when you see a range, the range is honest. It is not vagueness or hedging. It reflects a real distribution of outcomes among real people, and the useful question is not "what is the number" but "where in this range do I likely sit, and what should I do about it." That second question is the one a genetic counselor is trained to work through with you.

What if you have already been diagnosed?

If you are reading this after a cancer diagnosis, the framing of this post deserves a direct word.

Incomplete penetrance is not a suggestion that your cancer was avoidable, and it is not a comment on anything you did. Some people with a variant develop cancer and some do not, and which group a person lands in is not a reflection of effort, choices, or character.

Penetrance still matters for you, though, in a few concrete ways. It helps explain why your relatives who carry the same variant face elevated risk rather than certainty, which can make those conversations easier. It informs discussions about your own risk of an additional primary cancer, which may be part of your care going forward. And it is part of how your team interprets your result in the context of your family. Our post on genetic testing after a cancer diagnosis covers more of what a result can mean when you are already in treatment.

What this means for you

If you carry a hereditary cancer variant, the most useful thing you can do with a risk number is bring it to someone who can put it in context.

A genetic counselor can look at your specific gene, your specific variant, your family history, and your personal history, and translate a population statistic into something meaningful for you. More importantly, they can turn it into a plan: what screening makes sense, when it should start, what prevention options exist, and what your relatives should know.

A few resources from our blog that may help:

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