The Children Of Identical Twins Don't Have The Genetic Relationship You'd Expect

Identical twins have a knack for baffling the scientific community. From their inexplicable telepathic bonds (via Live Science) to controversial testimonials that they can feel each other's pain (per Daily Mail), the phenomenon of twindom is wrought with curiosity. According to the Chicago Tribune, Australian neuropsychiatrist Perminder Sachdev once referred to twins as "nature's experiment" a fascination that is fueled not by possible psychic bonds but something a lot more tangible and easier to quantify – DNA.

Unlike fraternal twins, who develop in two separate fertilized eggs and share approximately 50% of the same DNA (via Minnesota Center for Twin and Family Research), identical twins, who develop in the same egg after it splits in half, boast nearly 100% identical DNA. In fact, in a recent study reported by Smithsonian Magazine 38 of 300 identical twins were exactly the same genetically while the rest boasted only minor differentials.

Scientists have long been perplexed out how these close to completely identical genetic framework affects their lives. But you too, might do a doubletake when you find out how their children are related, at least from a genetic perspective. So, let's break this down. Let's say for example, identical twin sisters each have three children. Those children are cousins, aren't they? Nope. Not if you stare at their genes under a microscope.

The Guardian reports that children who come from one set of twins who do not marry another set of twins are socially regarded as cousins but scientifically categorized as half-siblings. This is because they will share as many DNA similarities as children who have the same dad but different moms or the same mom but different dads.

According to The Tech Interactive, half siblings share roughly 25% of the same DNA. This doesn't sound like a lot, but it's double the DNA similarities boasted by traditional cousins. While a rough estimate, it's also half the DNA shared by children with the same mom and dad, hence the term half-sibling.

The math on this is pretty simple. Traditional full siblings share 50% of their DNA, while identical twins have identical or near identical genetics, creating a 100% DNA match. So, when brothers or sisters have offspring, the level of shared DNA between the cousins drops down to about 12.5%, unless they are identical twins whose shared DNA was double that number from the start. Then those same cousins technically become brothers and sisters, though they might spend entire lifetimes not knowing it. 

Like most things twin, this is only the beginning. When identical twins marry identical twins, that's when the true duplexity unfolds...

Things get even trickier when identical twins marry identical twins, a phenomenon known as "quaternary marriage" that has happened approximately 250 times in recorded history (via Futurism). In other words, it's pretty rare, garnering lots of stares and attention, especially on a scientific level.

One of the most intriguing byproducts of quaternary marriage is the fact that children of these two sets of twins share about 50% of their DNA, making them genetically full siblings (per The Guardian). Not only do these full siblings who are called cousins share 50% of their DNA with each other, but they also share approximately as many genes with their aunts as they do with their mothers.

Twins, especially identical twins, are scientifically relevant for the nature vs. nurture argument which discusses how much of our personalities are predicated by genetic code. Since offspring of twins who marry twins is such a rarity, very little information is currently available on the topic.