He works alongside two of his former Roseanne cast members, Sara Gilbert, who during his role as David Healy played the role of his longtime girlfriend and later his wife, Darlene Conner, as well as Laurie Metcalf, formerly cast as the role of Rosanne Barr's fictionalized character's sister Jackie. Galecki is a cellist and has played cello on the show.
He has a younger brother, Nick, whom he describes as a mechanical genius in the automotive industry and a younger sister, Allison. During an interview with Zealand radio station ZM, Galecki jokingly recalled his childhood relationship with his mother.The actress recently told CBS Watch Magazine that the co-stars had a serious and lengthy relationship which has been a complete secret until now. It was such a huge part of my life and no one knew about it," the actress said.Cuoco, 24, and Galecki, 35, previously dated on the sitcom as well, as the characters Penny and Leonard.John Mark "Johnny" Galecki (born April 30, 1975) is a Belgian-born American actor.He is best known for his role as Leonard Hofstadter in the sitcom The Big Bang Theory since 2007 and as David Healy in the sitcom Roseanne from 1992 to 1997.He also appears in the music video for the Dave Matthews Band song "Satellite".