Credit: TLC

If Identical Twins Married Identical Twins, How Genetically Similar Would Their Children Be?

It might sound crazy to some people (maybe even taboo to others), but a pair of identical twins marrying identical twins is called a ‘quaternary marriage.’ Although rare, they do happen. In fact, there are about 250 marriages like this on record (The most famous of which, are Teyolla and Keyolla Loux and their husbands, Shawn and Eric Crow). Unsurprisingly, these couples may even go on to have identical twins of their own.

In either scenario, from a genealogical (not to mention, legal) point of view, any offspring would technically be considered first cousins, but their DNA would be hard to distinguish from that of full genetic siblings. Furthermore, it would be almost impossible to figure out who the biological parents are based on the DNA alone (of course, that explanation is too tidy. There are also other factors that might make formal identification possible).

How can this be? Think about it for one mind boggling moment…

Identical twins share the exact same genetic material, egg/sperm and all (while fraternal twins are conceived from a different egg and sperm). So in this case, the four “sets” of DNA are the same as only two sets.

A couple of identical couples recently doubled up for life.

Identical twins Brittany and Briana Deane first met identical twins Josh and Jeremy Salyers last August at The Twins Day Festival in Ohio, according to ABC News. A year later, the couples had their weddings, together, wearing the same wedding outfits, at this year's Twin Day Festival. Their weddings were documented for a TLC special called "Twins Marry Twins," scheduled to air in 2019.

The double Deane-Salyers weddings raise a question: If one set of identical twins marries another set of identical twins, how genetically similar would their children be? [Seeing Double: 8 Fascinating Facts About Twins]

"In theory, we would expect their children to be genetically as similar as full siblings are to each other," said Laura Almasy, a professor of genetics at the University of Pennsylvania. Siblings born to the same set of parents share about half their genes with one another.

For every child that's born, "it's like drawing a handful of marbles from a jar ― you get half of your DNA from mom and half from dad," Almasy told Live Science. Cousins don't draw from the same jar. But in this case, the children of the two sets of parents would be drawing from two separate, but identical jars.

"So, the expectation would be that they would share half their DNA like full siblings would," Almasy said. But just like typical siblings, "any particular pair of individuals could share more or less" of those genes.

What's more, the identical twin parents may not have completely identical DNA; instead, there may be slight differences in their genes that could have occurred in the womb or because of environmental factors throughout their lives. These slight differences can also influence how similar the babies would be to one another.

"Environment doesn't affect which genes [babies] get from their parents," Almasy said. But the environment in which the parents grew up could change how an inherited gene is expressed ― in other words, how it turned off and on, she added. For example, if one mother smoked throughout her life and the other didn't, both babies could receive the same gene, but the gene could express and influence the baby very differently.

Interestingly, the occurrence of identical twins marrying identical twins may not be so uncommon. Just recently, another set of identical twins, Krissie and Kassie Bevier, tied the knot in Michigan with two identical twin brothers, Zack and Nick Lewan, according to People magazine.

Originally published on Live Science.