At least, that's the theoretical definition. In reality, there are some folks who take a different view.
Some folks make up a lot of their heritage as they go along.
It is always amazing to me to listen to some folks expound on their family history from one occasion to another. Here in North Carolina, it is not uncommon to find families whose Scottish heritage takes on the trappings of royalty, depicting every clan as directly descended from Bonnie Prince Charlie himself. Other folks claim high-ranking descendants from various European families, African chieftains, and Asian princes. Ironically, these same folks say we should have stricter immigration laws. Some folks get deathly serious about their ancestors.