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You should take a DNA test. They're fun.
Also, if you're interested in learning more about your genealogy, I'd recommend https://www.reddit.com/r/Genealogy/
I know reddit has a bad rep, but their genealogy board is actually really good, and I've gotten a lot of really good info there.
Right now, though, given what you've said, I'd guess your grandfather was 3/4 Jewish, 1/4 Russian.
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