Last week, I read the story of two identical twin brothers born 20 minutes apart, who married two non-identical twin sisters born 20 minutes apart, who had baby daughters 20 minutes apart! That got me thinking. What is the likelihood of all those events? In a previous post, I explained why it is almost impossible for you to be born (if you didn’t read it, you can find it here). Now, my brain was spinning. What would happen if two identical twin brothers married twin identical sisters and had babies? This type of marriage is called “quaternary marriage” and, apparently, there are about 250 known such cases in the world.
Let’s consider this scenario. Identical twins have the exact same genetic material. Therefore, these twin brothers are genetically identical and so are the twin sisters. Now, one brother has a baby with one sister. And the other brother has a baby with the other sister. From a genetic perspective, these resulting baby cousins are siblings! Yes, you read it right. Cousins that are siblings genetically. Let’s take it one step further. If both sets of parents are genetically identical, there is a theoretical chance that both baby cousins could be identical too. How likely is that? Almost impossible. It is akin to asking “how likely is it that siblings born at different times from the same parents will be identical?” There is a chance that they could be but that chance is almost zero.
We have established that the cousins of a quaternary marriage are siblings genetically. What does this scenario mean to them? First, a DNA analysis could not distinguish who the parents of each baby are! Remember: the babies’ parents are identical. When a DNA sample from one baby is tested against the two sets of parents, it will match to both. Second, according to civil law in most countries, cousins may marry each other. But what would the law say if the cousins are also siblings? I believe there is no legal precedent anywhere in the world to answer that question.
I have never heard of cousins who are identical twins (forgive me for the misleading title of this post!). They have probably never existed nor will they ever exist since environmental factors and biological processes called “epigenetics” and “genetic recombination” take care of making them look different. But cousins who are siblings genetically do exist!
Photo caption: Identical twin sisters Diane and Darlene Nettemeier met and married identical twin brothers Craig and Mark Sanders. The couples now have 5 children between them (all siblings genetically). Diane and Craig even had identical twin boys!