Determining ethnicity according to ancestry My sister, cousins, and other relatives speculate that my mom's grandmother was Jewish (for various reasons). I've heard people say at times things like, for example, "I'm 50% German" or "I'm 25% Swedish." This has to do with their ancestry. How exactly is this calculated? For example, if my mother's paternal grandmother was indeed 100% Jewish, what would that make me and my siblings, and how is it calculated?

The rule for this style of accounting is straightforward: a person is 50% what their mother was and 50% what their father was.

Sometimes one will want to simplify a result. For instance, if your mother is Russian and your father is half Russian and half Chinese, then you are 50% Russian plus 50% half-Russian-half-Chinese, which you can simplify like this:


25% = 50% x 50% Chinese (father)
25% = 50% x 50% Russian (father)
50% = 50% x 100% Russian (mother)


simplifies to


25% = 50% x 50% Chinese (father)
75% = 25% + 50% = (50%*50%+50%) Russian (father plus mother)


So under the presumption that "being Russian", "being Jewish", etc. is inherited in this way, if your great-grandmother was Jewish, then you are 50%x50%x50% = 12.5% Jewish because of her, plus whatever percentage Jewish you inherit from your other seven great-grandparents.