The Jewish people have what we (Jews) call “close knit communities” which result in “intercommunal breeding”. It’s not nearly as bad as it sounds, but it does increase the concentration of certain genetic traits. You can see examples of this in purebred animals. Pugs, for example, didn’t always used to look like their faces got smooshed up against the window and as a result of breeding that specific trait, nearly all purebred pugs have serious health issues, especially when it comes to breathing.