Do babies born between two cousins actually have a higher chance of having birth defects? Understanding basic genetic principles will help with this question.

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Following is a transcript of the video:

What did Queen Victoria, Charles Darwin, and Albert Einstein have in common? 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 lead 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-grandparents.

Genetically speaking that means they shared 0.20% of their DNA. Not much when you compare it to third(0.78%), second(3.13%), and especially first cousins(12.5%). And, the more DNA you share, the greater the 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’re unaffected. Instead, you’re what’s called a carrier.

Now, if one carrier mates 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 vs. just one? What if one of their children is a carrier vs. 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 4-7%. For the general population, it’s 3-4%.

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 I 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.