How rare is it to find a person with 100% genetics of only one culture?

I was thinking how back in the days to be a made man in the Italian mafia you had to be 100% Italian which I can only assume that means the whole family tree would have to be from that culture, with the ancestry tests they have today it seems like everyone has a mix of something so how common is it? πŸ€”
How rare is it to find a person with 100% genetics of only one culture?
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