You can’t have a genre without expectations and you can’t have expectations without stereotypes.
A genre is a style of category of art, music or literature.
An expectation is a strong belief that something will happen or be the case.
A stereotype is a widely held but fixed and oversimplified image or idea of a particular type of person or thing.
For example; romance is a genre, the expectations are a sympathetic heroine, a strong and irresistible hero, emotional tension and a happy ending (it would help if the plot was believable) and the stereotypes are the bad boy and the good, shy and ‘nerdy’ girl or the damsel in distress and the prince that saves her. I.e. Pride and Prejudice, Me Before You, Cinderella and Beauty and the Beast.
For a genre to work the reader needs to have expectations about the story and the expectations come from the stereotypes that have been placed on the genre. The stereotypes are what makes the expectations of the genre and the expectations are why the genre works.