I have a disease that is autosomal dominant (and absolutely sucks.) That means only one gene (the one I got from Dad) was needed to give me the disease.

Now I have 2 of these genes. Everybody does. You get one from your mother and one from your father. So I got the bad one from Dad, and a good one from my mother.

It’s a super rare disease! If I nail somebody at random in the population, they have 2 good genes they can contribute. My one bad one and one good one flip a coin; our kids could have a bad one from me (and die young) or a good one from me (and be fine!)

But if I nail a brother who had the bad gene, there’s a coin flip for both of us. Even if I give a good gene, he might give a bad one! Way more likely the bad thing happens.

(So, so grateful none of my brothers have it so I don’t have to get all squicked imagining that but it makes my point.)

Not all bad things are as easy to see as my disease. Lots of them “hide” until you get two bad things. But it’s more likely two siblings have similar hidden bad things, which makes it more likely the bad things will show up in their child.