The inherited gene mutations linked to breast cancer follow an autosomal dominant inheritance pattern, meaning you only need to inherit one copy of the mutated gene (from one parent) to have an increased risk. But the biology at the cellular level is actually recessive. This distinction confuses a lot of people, and it’s worth understanding because it changes how you think about family risk.
Dominant Inheritance, Recessive Biology
The major breast cancer genes, BRCA1 and BRCA2, are tumor suppressor genes. Their job is to help repair damaged DNA and prevent cells from growing out of control. When one copy is mutated, the other healthy copy can still do the job. So at the cellular level, these mutations are recessive: both copies need to be knocked out before a cell loses its ability to suppress tumors.
Here’s where it gets interesting. In families, these mutations look dominant. If a parent carries a BRCA1 or BRCA2 mutation, each child has a 50% chance of inheriting it, and inheriting just that one copy is enough to significantly raise cancer risk. The reason is something cancer biologists call the “two-hit” model. People who inherit one broken copy start life with only one functioning copy in every cell. Over a lifetime, random damage to that second copy in just one cell is enough to trigger cancer. Someone without an inherited mutation would need both copies knocked out by chance in the same cell, which is far less likely.
So the answer depends on which level you’re asking about. In a family tree, breast cancer susceptibility genes behave as dominant traits. Inside a single cell, they behave as recessive ones.
How Much Risk One Copy Carries
Inheriting a single mutated copy of BRCA1 or BRCA2 doesn’t guarantee breast cancer, but the lifetime numbers are high. Cumulative risk estimates by age 80 are 72% for BRCA1 carriers and 69% for BRCA2 carriers. More than 60% of women who inherit a harmful change in either gene will develop breast cancer during their lifetime. Men with these mutations face lower but still elevated risks: roughly 0.2% to 1.2% for BRCA1 carriers and 1.8% to 7.1% for BRCA2 carriers by age 70.
The fact that not every carrier develops cancer is what geneticists call incomplete penetrance. The inherited mutation is necessary but not sufficient on its own. That second “hit,” the random loss of the remaining good copy in a breast cell, still has to happen. Environmental factors, hormonal exposure, and plain chance all influence whether and when that occurs.
Other Genes Follow the Same Pattern
BRCA1 and BRCA2 get the most attention, but several other genes linked to breast cancer follow the same autosomal dominant inheritance pattern. PALB2, CHEK2, and ATM are classified as intermediate-risk genes. They carry lower risk than the BRCA mutations but still meaningful numbers: predicted average breast cancer risk by age 80 is about 50% for PALB2 carriers, 30% for CHEK2, and 28% for ATM. Like BRCA mutations, inheriting one copy from one parent is enough to elevate risk, and each child of a carrier has a 50/50 chance of inheriting the variant.
Most Breast Cancer Is Not Inherited
Despite the strong association between these gene mutations and cancer, hereditary mutations in BRCA1, BRCA2, and related genes account for only about 5% to 10% of all breast cancer cases. The vast majority of breast cancers are sporadic, meaning they arise from mutations that accumulate in breast cells over a person’s lifetime rather than being passed down from a parent. In sporadic cases, both copies of a tumor suppressor gene are damaged by random events, with no inherited head start.
This is an important distinction for anyone worried about family history. Having a close relative with breast cancer does increase your statistical risk, but most of that risk is not driven by a single dominant gene mutation. Shared lifestyle factors, hormonal patterns, and common genetic variants that each contribute a small amount of risk also play roles. The high-penetrance inherited mutations that follow clear dominant inheritance patterns are rare in the general population.
What This Means for Families
If a known BRCA1, BRCA2, PALB2, CHEK2, or ATM mutation runs in your family, the inheritance math is straightforward. Each first-degree relative of a carrier has a 50% chance of carrying the same mutation. The mutation doesn’t skip generations in a hidden way like a classically recessive trait (such as cystic fibrosis) would. If you didn’t inherit the mutation, you can’t pass it on, and your breast cancer risk returns to the general population baseline.
Genetic testing can identify whether you carry one of these mutations. Testing is most informative when it starts with a family member who has had cancer, because a negative result in that person means the family’s cancer likely isn’t driven by one of these known genes. A positive result in a relative who has had cancer gives other family members a specific mutation to test for, making their own results far more definitive.