Susan

Susan (last name unknown) is the of Penny and  of Wyatt. She also has two other, one named Randall and a second girl who we have never met. She lives with her outside of Omaha, Nebraska on their small farm where they raised their three children.

She is the only mother of any members of the gang who has never been seen on the show, except Stuart Bloom's mother who is also only mentioned and Emily Sweeney's mother hasn't been seen or mentioned.

In "The Flaming Spittoon Acquisition", Sheldon implies that she's fat (as well as in The Pulled Groin Extrapolation" where he states he has seen pictures of her and sarcastically encourages Penny to keep eating) and Penny reveals that her mother smoked when she was pregnant with her.

In "The Transporter Malfunction", Leonard and Sheldon told Penny they didn't want to open the Star Trek Transporters she had just bought them because once you open the box, it loses its value. Penny relied, "Yeah, yeah, my mom gave me the same lecture about my virginity. I gotta tell you, it was a lot more fun when I took it out and played with it."

In "The Status Quo Combustion" Penny calls her mother to tell her about her engagement to Leonard. She denies that she is pregnant and agrees that it is a first for their family.

In "The First Pitch Insufficiency", Amy wants to know when Penny and Leonard are getting married. They haven't set a date and Penny wants to wait long enough to prove to her mother that she's not pregnant.

In "The Bachelor Party Corrosion", Penny tells her father that she eloped with Leonard. Her mother was out with Penny's brother who was celebrating being sober for one month and had received his 10th chip on account of being sober for a month. Her mother was going to make her son a little necklace to commemorate the occasion.

Trivia

 * Kaley Cuoco had stated that she would like to have Lisa Kudrow play Penny's motherLisa_Kudrow_2.jpg.
 * It seems that she has already met Leonard off-screen because in "The Bachelor Party Corrosion" while Penny, Bernadette and Amy are discussing Penny waiting to tell her parents she got married to Leonard, Amy says "Well, it's kind of sad that they don't know. I mean, they love Leonard." referring to Penny's parents.
 * According to a July 2016 article on the TBBT Facebook page, actress has been cast as Penny's mother, Susan, and will be seen in the Season 10 premiere. This will be a reunion of sorts for Sagal and Cuoco, who played mother and daughter on the sitcom . Kaley has sated that Katey is her "TV mom".