It's kind of a pun between the "drop" in a Q drop, and the "one drop of blood" rule quoted by many politicians to determine whether a person was black or white. (That was back when there were laws discriminating between blacks and whites, and so it was used by some as a bright line, to avoid dealing with mixed-race people. Taught in some history classes in high school.)

So to invert the one drop idea, by implication, the rabbit has at least one drop of white rabbit to make it grey. Explaining it makes it far less clever. But now you know!

And with Q of course, one drop is all it takes to get hooked.