Though she was never introduced in Star Trek: The Original Series, Dr. McCoy's daughter, Joanna, was mentioned in several Star Trek novels. And had D.C. Fontana's script, "Joanna," been produced, we would have met her onscreen, but the episode showed a darker side of Dr. Leonard "Bones" McCoy as revealed by Forgottentrek.
In the episode, which is much like "The Way to Eden," Dr. McCoy is shocked to discover that his daughter has been traveling with Dr. Sevrin and his group of "flower children." He was under the impression that she had been studying to be a nurse for the past three years. A furious conversation ensuies, and it's revealed that not only did Dr. McCoy walk out on his wife and daughter, but he and Joanna have only seen each other three times in her entire life.
The script later reveals why Dr. McCoy left his wife, and, as a result, Joanna's mother threw her out for being too much like her father. This is obviously meant to be a bonding moment with the two characters, but it doesn't read that way on paper.
When Dr. McCoy discovers that Captain Kirk has taken an interest in his daughter, he warns him off but not because she's his daughter. Instead he says that Joanna is a witch like her mother and that's why he left. He added that she was no good and would cut his heart out and carry it around in a jar. Pretty harsh words from a father who had only seen his daughter three times in her life.
Though at the end of the script, Dr. McCoy and his daughter partially reconcile, it's a relief that this script wasn't produced. Though Dr. McCoy could be gruff and curmudgeonly, he was never cruel. And there would have been no way to create this episode without him appearing to be that.