The Propagation Proposition Season 12, Episode 12 -  Aired January 10, 2019

The Propagation Proposition

Penny's ex-boyfriend, Zack (Brian Thomas Smith), and his wife want a baby, but when Zack is infertile, they ask Leonard for help. Also, Koothrappali attempts to mend things with his ex-fiancée, Anu.

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Guest Stars: Brian George as Dr. Koothrappali, Brian Thomas Smith as Zack, Rati Gupta as Anu, Lindsey Kraft as Marissa, Shay Ali as Greg, Chip Chinery as Bob

Writers: Chuck Lorre (Story), Steve Holland (Story), Jeremy Howe (Story), Maria Ferrari (Teleplay), Dave Goetsch (Teleplay), Eric Kaplan (Teleplay)

Director: Mark Cendrowski

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