They all married their first cousins.

You'd think Darwin of all people would know better.

After all, mating with a close relative passes on bad genes that leads to deadly genetic mutations.

Right?

Today, marrying your first cousin is illegal in 24 US states.

But for most of Western history, people had to marry whoever lived nearby, which oftentimes meant marrying within the extended family.

In fact, between 1650 -1850, the average married couple was fourth cousins.

So they had the same great, great, great, great grandparents.

Genetically speaking, that means they shared 0.2% of their DNA.

Not much when you compare it to 3rd, 2nd and especially first cousins.

And the more DNA you share, the greater chance your offspring will have a genetic disease like cystic fibrosis or sickle-cell anemia.

But here's the thing, you don't have to be sick in order to give your child a genetic disease.

Take cystic fibrosis. It's caused by a defect in the CFTR gene, but you need two copies of the defective gene to actually get the disease.

So if you only have one defective copy, you are unaffected. Instead, you are what's called a carrier.

Now, if one carrier meets with a non-carrier, there's no risk of the kids getting sick.

But when both parents carry a defective copy of CFTR, then the kids have a 25% chance of inheriting two copies of the gene and having the disease.

So to see how dangerous it is to marry your first cousin, we need to calculate the chances that two first cousins both carry a copy of the same genetic disease.

Since they share a set of grandparents, we'll start there.

Now, it becomes a game of what ifs: what if both grandparents are carriers versus just one?

What if one of their Children is a carrier versus none at all?

And what if those Children marry other carriers or not?

It can get very complicated very quickly.

But scientists have crunched the numbers and it turns out the risk that the cousins have a kid who inherits a genetic disease is 4 to 7%.

For the general population, it's 3% to 4%.

So, not a big deal. Right?

Here's the catch, that's the odds for one genetic disease.

But there are thousands that could be hiding in your family tree.

Plus, if your kids also marry their first cousins and their kids marry their first cousins, it's a recipe for disaster,

because instead of introducing new, potentially helpful genes into the family gene pool, you're recycling the old and possibly dangerous ones.

Take Emperor Ferdinand the first of Austria, his parents were first cousins twice over.

And when he was born, he wasn't especially healthy.

So as far as marrying your cousin is concerned, you shouldn't make it a family tradition.

What interesting things have you found out in your family tree?

Let us know in the comments.