Tom Riddle, Jr. var ett halvblod, men han passar in i Gaunt-familjen genom sin mor (betonar min):
"So Merope," said Harry, leaning forward in his chair and staring at Dumbledore, "so Merope was... Sir, does that mean she was... Voldemort's mother?"
"It does," said Dumbledore. "And it so happens that we also had a glimpse of Voldemort's father. I wonder whether you noticed?"
"The Muggle Morfin attacked? The man on the horse?"
"Very good indeed," said Dumbledore, beaming. "Yes, that was Tom Riddle senior, the handsome Muggle who used to go riding past the Gaunt cottage and for whom Merope Gaunt cherished a secret, burning passion."
Half-Blood Prince Chapter 10: "The House of Gaunt"