Denise Crosby, unhappy with her role of Lt. Tasha Yar on Star Trek: The Next Generation, asked to be let out of the series in its first season. While the other actors decided to wait to see what would happen with their characters, she didn't want to be stuck on the bridge of the Enterprise just saying "aye-aye" all the time. In a recent interview by IGN, Crosby noted that her decision to leave threw things into a bit of a tailspin for the freshman sci-fi series.
Gates McFadden, who portrayed Dr. Beverly Crusher, during the first season of The Next Generation as well as seasons three throughseven, was fired at the end of the first season, and Marina Sirtis thought she was going to be fired as well. That would have left The Next Generation with no female characters. So Sirtis remained, and Diana Muldaur was hired as the second season's doctor, although she only remained for one season.
Crosby said she had no intention of "wreaking havoc" and that she just wanted someone to tell her who Tasha Yar was going to be, what was going to happen with her character. She knew being on the series was an incredible opportunity, but so far, her character hadn't gone anywhere.
"And now they're like, ‘Oh my God, there's no … women. So now we’ve got to keep Marina and Gates we'll recast. … It kind of wreaked havoc. That wasn't my intention. My intention was to get somebody in the room and tell me, ‘What is this going to be? What is this character?’ It's such an incredible opportunity. You have so much here, but I'm not going to just be the token hot blonde on the show. But they had a ’60s mentality. It was all these old white dudes in the room until, God love them, until Gene passed. And it shifted. There was a shift when [showrunner] Michael Piller came onto the show [in Season 3] and things changed."Denise Crosby
Crosby's character was never replaced. Instead, Michael Dorn's Worf took the position as Chief of Security of the Enterprise, and it opened up a wealth of opportunities for him. Once Crosby left, The Next Generation continued with two main female characters for the remainder of the series, with McFadden returning in the third season.