In the episode "Let That Be Your Last Battlefield", the Enterprise pick up a law enforcement officer (Bele) and a radical freedom fighter (Lokai) who have been involved in a centuries long chase. Making the least subtle point in television history about the evils of racism, the pair eventually make it home to see that both sides in their war have destroyed themselves. What was the distinguishing biological difference between the Bele and Lokai? (we'll be lenient)
Bele was black on his right, and white on his left, while Lokai was the reverse.