When you hear the word penetrance, you might think about sex. But in genetics penetrance refers to how strong your genes are acting.

Imagine someone says a gene has 20% penetrance. What that means is that 20% of the people who carry the gene, will actually show the effects of that gene. So when we say a gene associated with breast cancer has incomplete penetrance. It means that not everyone with that gene will actually get breast cancer.

Why is that? Well it’s because genes don’t stand alone- they interact. And because we all have slightly different genes, they may behave slightly differently in each person.

It’s like the guy who’s the life of the party, getting attention from every person in the room. That’s like a gene showing it’s affect. But that same guy at a different party with different people may be a wall-flower. That’s like a gene having no affect. His behaviour, like every genes behaviour, depends on who he’s interacting with. In the right setting that wall flower becomes the party animal. How often he’s the party animal is like the penetrance of a gene. So actually penetrance may have more to do with sex than we thought.