royalty
Explanation
The Joke
The comic is set in a medieval or fantasy-style throne room. A guard or advisor addresses a ruler, saying something like "Sire! You walk among us as a commoner! I can see at once that you are from a noble bloodline, superior generations, untainted by outsiders." The king responds simply: "How did you know?" The joke is that the advisor's description of "untainted by outsiders" and "superior generations" is really just a polite way of describing inbreeding, which was historically rampant among European royal families who married within a small pool of other royals to preserve bloodlines.
The humor is that the advisor frames what is essentially a genetic bottleneck as a mark of distinction and superiority. The king, rather than being offended, takes it as a compliment, not realizing (or not caring) that he is being identified by the visible signs of a severely limited gene pool.
The Humor
The comedy works on the historical irony that European royalty's obsession with "pure" bloodlines led to well-documented genetic problems -- the Habsburg jaw being perhaps the most famous example. What the nobility considered their greatest strength (genetic purity and exclusivity) was actually their greatest weakness. The advisor's flowery language dresses up a medical diagnosis as flattery, and the king's oblivious pride makes the irony even sharper.