‘Okay, I had no idea that lemons and grapefruit are actually hybrid mixes of other fruits. How did I get to age 31 and miss this? Better yet, both citruses were born accidentally, of illicit love affairs not arranged by human hands. Lemons are the love child of citron and orange. Grapefruit the natural daughter of Asian pomelo and Barbados sweet orange.’ (Boing Boing).
I only recently learned this myself after starting to eat pomelos and wanting to research what relationship they have to grapefruit.