<Table> <Tr> <Td> </Td> <Td> This section of a biography of a living person needs additional citations for verification . Please help by adding reliable sources . Contentious material about living persons that is unsourced or poorly sourced must be removed immediately, especially if potentially libelous or harmful . (May 2015) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section of a biography of a living person needs additional citations for verification . Please help by adding reliable sources . Contentious material about living persons that is unsourced or poorly sourced must be removed immediately, especially if potentially libelous or harmful . (May 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> Galecki made his acting debut in the 1987 CBS miniseries Murder Ordained with JoBeth Williams and future Roseanne co-star John Goodman . In 1989, Galecki portrayed Rusty Griswold in National Lampoon's Christmas Vacation . In 1990, he was cast as Danny Nash, the son of Robert Urich's lead character, on the NBC comedy American Dreamer; the following season, he was a regular cast member on the ABC sitcom Billy, a spin - off from Head of the Class . He appeared in one episode of Blossom in 1991, opposite his future The Big Bang Theory co-star Mayim Bialik . </P> <P> During his run on Billy, Galecki began making guest appearances on the hit ABC sitcom Roseanne as the younger brother of Mark Healy (Glenn Quinn), who began a relationship with Darlene Conner (Sara Gilbert). Introduced in his first appearance as Kevin Healy, his name was soon changed to David . After a few more guest shots, Galecki was named a permanent Roseanne cast member from the fall of 1992 onward, after the cancellation of Billy . He would remain in the role of David Healy until the end of Roseanne's run in 1997, with the character eventually marrying Darlene and fathering a child with her . </P>

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