You may have heard the story of the identical twin sisters, Brittany and Briana Deane, who met, dated, and married identical twin brothers, Josh and Jeremy Salyers. They tied the knot in a joint ceremony (a quarternary marriage), wearing matching outfits earlier this month.
The story generated a lot of headlines but it is not nearly as uncommon as you might think – indeed, there are around 250 cases of identical twins marrying identical twins on record, reports Futurism. That includes Teyolla and Keyolla Loux and Shawn and Eric Crow, Krissie and Kassie Bevier and Zack and Nick Lewan, and Diane and Darlene Nettemeier and Craig and Mark Sanders, who like Brittany, Briana, Josh, and Jeremy met at the Twins Day Festival in Twinsburg, Ohio. (This is an annual occasion where twins gather, usually sporting some sort of matching costume – only in America.)
Perhaps it's not all that surprising, either. Identical twins have a lot in common – and presumably, that includes their choice of partner. But, you might wonder, what does this level of genetic similarity mean for their children?
Legally-speaking, the children of one set of parents are first cousins to the children of the second set of parents. Yet biologically-speaking, they have much more in common with siblings, which means the difference in DNA would be comparable to sisters and brothers. This is because they come from a very similar (if not the exact same) gene pool.
Every time a sperm combines with an egg, half the mom's DNA is mixing with half the dad's DNA. The combination of DNA will be different at each conception but the DNA pot is always the same. On average, genetic siblings are expected to share around half their DNA with one another but this is just an average – it may be more or it may be less. If both your moms and dads are identical twins, you are effectively selecting from the same DNA pot. Ipso facto, you are genetically comparable to genetic siblings.
But, while the gene pool is remarkably similar, it is not entirely identical. This is because the DNA carried by sperm and egg can change after conception, in the womb and post-birth. (There may also be mutations in the sperm and egg cells that affect the DNA before conception.) For example, smoking, exercise, and diet can all change the way the genes we are born with express themselves. Differences in lifestyle choices between identical twins can mean differences in the way their DNA expresses itself, so the children from each set of identical twin parents could be selecting genes from a (slightly) different DNA pot to begin with.
Finally, there is the process of "genetic recombination", which is a unique type of cell division (meiosis) that occurs when sperm and egg cells form so that each has the correct number of chromosomes. The chromosome pairs crush together and crossover in a way that ensures every sperm and every egg is like none other – imagine each gene as candy at a pick 'n' mix stand and the chromosome pair is a random combination of that candy. This prevents any sperm and egg combo being identical to one another.
So, there you have it. If Jeremy and Briana, and Joshua and Brittany were to have children, there is no chance those children would be identical but they would be almost as close genetically as siblings. What's more, there is a higher than average chance that some of those children would be identical twins themselves.