A good piece on the background of this concept. For those new to this, this passage might be worth highlighting:
“As historians have documented, it also influenced the ideology of Nazi Germany. Translated into law, it produced such horrifying artifacts as Virginia’s 1924 anti-miscegenation act, passed with the aid of the eugenicist Anglo-Saxon Clubs. The law required all babies to be classified as “white” or “colored” and made it a felony to “misrepresent” your racial background. The Nazi jurists studying American race laws in the 1930s thought such “one drop” rules were a bit too strict.”