Do all descendants of your great-grandfather all have similar Jewish DNA markers?  If they are only seen in one of his children's line it suggests differing parentage.

Every child inherits 50% of DNA from each parent, but each is a random assortment of the parent's DNA. So you and a sibling only share 25%. That is why organ transplant Dr. look at parents or children first for a good match.

So all descendants of a great-grandparent will not have the same Jewish DNA markers. There are a lot of markers checked, so DNA testing will probably find some, but not all the same.

Sally Bruckheimer
Molecular Biologist who knew this in high school
Princeton, NJ