i have a made up theory i dont know how to prove. it probably holds no water but its interesting to think about. It is that there is a parasite in the jew gut biome. That would be why a person can only be ethnically jewish if they come from a jewish mother, and why the father is less important unless one is trying to make a full breed jew. Because, the mother passes on the gut biome culture of hers to the child when its in the womb. of course all the inbreeding doesnt help, those genes can be passed down from both the father or mother.