Genea-Musings's Randy Seaver pointed to a Wizard of Id cartoon indicating a possible "non-paternity even" - as genetic genealogists term it.
I went over to the Comics.com site and plugged in some appropriate keywords (family history, ancestry, genealogy). "Family history" turned up more than 2000 possibilities (some right on target, others more far-fetched).
I didn't have time to go through all of them, but there are many good ones.
You can email, share or embed the cartoons.