In a novel I'm reading, a character mentioned being "one-third French" and two-thirds something else. My instinct is that you can't actually be one-third, or two-thirds, anything. Doesn't your genealogy have to come in halves and quarters and eighths and so on? You have two parents, four grandparents, eight great-grandparents, etc. So is it actually possible to be "one-third" any given ethnic group?
Math people, enlighten us.
If my instincts are right, please add "I'm one-third [blank]" to the official list of fallacious things your characters should never say.