Royal Blood, Birth Defects and Marriage in Circassians

It seems like a tradition in Circassians (Çerkez) that they can only marry with someone who doesn't share a relative with them back through seven generations. A common ancestor found within seven generations is considered to be a sign of brother and sisterhood. They are known to value family ties, and many family have their own coat of arms. There is a knowledge of the attributes of each of the families whom to (they call) "give" a daughter to or not (1).

Circassian flag

I was wondering what may lead to this strict understanding of brother and sisterhood in Circassians, whereas in many traditional societies marriage with a relatively close relative is commonly observed.

Maybe it's because of the fact that they are looking for the optimum trade-off between being in a royal family and also giving birth to a healthy child. Maybe they know it instinctively that marriages within a closed community can raise the chance of birth defects.

There were many instances of where royalty was preferred over genetic well-being and lead to the catastrophe of many royal families with genetic diseases. The Russian crown had to deal with haemophilia and there were many more examples such as the English (porphyria) and the Spanish crown that led to the demise of itself by a weak heir and inevitably contributed its share to the Spanish Civil War.

Sources

1. Personal contact with Çiğdem Kılıçtaş