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At SciShow, we ask the tough questions. Today we explore the answer to the question "if identical twin brothers married identical twin sisters, would their offspring be identical?"

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We here at SciShow like to ask the tough questions. For instance, if identical twin brothers married identical twin sisters, would their offspring be identical? Because you know identical twins are genetic duplicates of each other, so would their kids be identical too? These are the kinds of questions that keep me up at night. Or at least they would if I didn't have the internet, and so here on the internet I am going to tell you the answer is: No. If identical twins mated with another set of identical twins their children would super definitely not be identical. Allow me to explain.

Identical or monozygotic twins come from the same fertilized egg, the very same zygote, since both siblings come from that single cell they have exactly the same DNA. Fraternal or dizygotic twins usually form when two different eggs are fertilized by two different sperm, and are implanted in the wall of the uterus at the same time, so even though they're twins their DNA is no more similar than my brothers and mine, and we're seven years apart. So if two pairs of monozygotic twins had kids with each other, the zygote that came from each couple would be different, just like the sperm and eggs that made my brother and me different.

This is due to genetic recombination, a special gene exchange process that happens only during meiosis, the type of cell division that takes place when your body is making sex cells whether they're sperm or eggs. As a result of recombination, chromosomes get swapped around at random as your individual sex cells are made, and the likelihood that genes will recombine exactly the same way twice in one persons sex cells is really small, about the same as two parents having two identical children that aren't twins. 

This all happens for a reason without recombination we'd basically be clones. We'd either be identical clones of our parents or half of our cells would be clones of our fathers, while the other half would be clones of our mothers, and that would be weird. What's more we'd just end up with all the bad genetic combinations that every one of our ancestors had, like for congenital diseases and stuff and no one wants that.

Instead, especially in more complex animals, nature tends to abhor cloning. Genetic diversity is what allows species to evolve, it's how we acquire traits that make us more fit, allowing us to adapt to our environment, to keep pace with pathogens and parasites and basically stay on top of the heap.

So yeah, each of us is a unique and beautiful snowflake, sorry. But the twins mating with twins is not without it's scientific fascinations, for instance, the offspring of two set of twins would be more genetically similar than most first cousins are to each other, in fact they'd be just as similar as full siblings are to one another, so they'd have that going for them, and interestingly enough identical twins are more likely to sire identical twins themselves. So what if the two pairs of twins each give birth to identical twins and then those twins all married each other. Well I don't have time to do the math for you but I'm pretty sure that's not legal in a lot of states.  

Thanks for watching this episode of SciShow, if you have any questions or suggestions we're on facebook or twitter and of course down in the comments below. If you want to continue getting smarter with us, head on over to and subscribe.

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