It is revealed that Sheldon's sister, Missy, is actually his twin.
Leonard: So, how do you two know each other?
Missy: He once spent nine months with my legs wrapped around his head.
Leonard: Excuse me?
Sheldon: She's my twin sister. She thinks she's funny, but frankly, I've never been able to see it.
Missy: That's because you have no measurable sense of humor, Shelly.
Sheldon: How exactly would one measure a sense of humor? A 'humormometer'?
Sheldon: You see, I'm a superior genetic mutation, an improvement on the existing mediocre stock.
Missy: And what do you mean, "mediocre stock"?
Sheldon: That would be you.
Missy: (to Sheldon) Okay, I'm not even going to ask why you're pimpin' me out for cheese, but why do you care who I sleep with?
Missy: Shelly, can I speak to you a minute—alone?
Sheldon: Why does everyone suddenly want to talk to me alone? Usually no one wants to be alone with me.
Howard: I'd kill my rabbi with a pork chop to be with your sister.
Penny: Sheldon, why are you ignoring your sister?
Sheldon: I'm not ignoring my sister. I'm ignoring all of you.
Howard: Okay, you two have to back off.
Raj: Why should I back off? You back off, dude.
Leonard: Excuse me, this is my apartment, and she's my roommate's sister.
Howard: So what? You've already got Penny.
Leonard: How do I have Penny? In what universe do I have Penny?
Howard: So, I can have Penny?
Leonard: Hell, no!
Raj: Missy, do you enjoy pajamas?
Missy: I guess.
Raj: We Indians invented them. You're welcome.
Howard: Yeah, well, my people invented circumcision.
Howard: (imitating Raj) I'm a fancy Indian man. We invented pajamas.
Raj: (imitating Howard) Hey, look at me, I don't have a foreskin!
(About Sheldon's sister)
Leonard: Maybe she's his lawyer.
Howard: She's free to examine my briefs.
Howard: I know, I'm disgusting. I should be punished... by her.
The German episode title is "Sheldon 2.0". The French title is "La Sœur jumelle", meaning "The Twin Sister". The Italian title is "Il fattore sorella gemella", meaning "The Twin Sister Factor". The Spansih title is "La indeterminación de la chuleta de cerdo", an exact translation. The Mexican title is "La indeterminación de la chica", meaning "The Girl Indeterminacy".
International Airdates: Turkey: December 23, 2008 on CNBC-e; Germany: September 19, 2009 on ProSieben; Slovakia: September 8, 2011 on Markiza
Alternate episode title: The Shiksa Indeterminacy.
This episode's end titles has Chuck Lorre's Vanity Card #208, an audio recording.
When Raj asks Missy "...if you wonder, wonder, who wrote the book of love," he is borrowing lyrics from the 1957 song, "Book of Love", by the Monotones.