Robert "Robbie" Scott Johnson is the son of David Johnson and Anne Johnson , the stepson of David's former assistant, the younger maternal half-brother of Amy Juergens and Ashley Juergens, the older paternal half-brother of Twin 1 and Twin 2, and the uncle to Amy's son [[John Underwood
Although Robbie does not resemble any of his immediate family, he does take after his maternal grandmother, Mimsy, who also has blonde hair and blue eyes. George thought Robbie was David's son until Anne told him that the dates of conception and David's near medical impossibility of being able to have biological children made her sure that Robbie was his son. However, when George saw Robbie didn't look like him, he became skeptical, but eventually brushed it aside.
When Anne discovered that David later had twin daughters, she became suspicious of Robbie's paternity. In the episode It's a Miracle, George had already taken a paternity test and he confirmed that David Johnson was Robbie's biological father.