You share a set of great-great-grandparents with your third cousin, but do not have the same great-grandparents.
I’m taking more about the TV plots where our main character discovers the person they’re newly dating and, presumably, very excited about is a some sort of second or third-ish cousin.
Think ’s episode “The Head and The Hair.” You know, the episode where Liz is dating the incredibly hot, successful TV personality, but they have to immediately break up upon finding out that Liz’s Great-Aunt Dolly is The Hair’s Grandmother’s Cousin?
Or, put another way, your great-grandparents were siblings. Assuming you’re between twenty and thirty years old, the common ancestor between you and your date was born sometime after the Civil War but before the turn of the 20th century. According to this article, you might have more genetic material in common with people whom you form relationships than those you are born into: After analyzing almost 1.5 million markers of gene variations, the researchers found that pairs of friends had the same level of genetic relation as people did with a fourth cousin, or a great-great-great grandfather, which translates to about 1 percent of the human genome.
Or, to make it blunt, you and your date have a great-great-grandparent in common. And while time itself does not dilute the gene-pool, basic mating does. So all those best-friends-turned-lovers relationships just got weird. If you and a close relative had both decided not to have children, society would not smile on your union just because it won’t produce an X-child.
I’m less surprised by the fact that Liz and the Hair were, at closest, second cousins than I am by the fact that the Hair had a framed photo of his Grandmother’s cousin in his apartment. I mean, I understand that from a social perspective, sure, you could identify a person who is somehow vaguely related to you.