In season one episode 12 - "Martha Complex" - Tobias mentions that he and Lindsay were unable to conceive, yet they have a daughter - Maeby. Since Tobias and Lindsay were apparently unable to conceive, does this mean that Maeby is adopted?
All is revealed in the next episode, "Beef Consommé". George Michael wonders about Maeby's paternity, and is told by Lucille that the family spent $130,000 in fertility treatments that eventually enabled Lindsay to conceive Maeby.
Source: Arrested Development wiki