The cultural assumption is that African-American (blacks) people in the early 1900's were treated poorly by Anglo Saxon American (whites) because they were considered a minority group. The author presents this cultural assumption through the informal and very mean language used to "Crooks" a negro stable buck.
An example of when Crooks is treated unfairly is how he is isolated from the other workers on the ranch. His bed is in a little shed that is off the side of the barn compared to the other ranch workers who all sleep together in a bunkhouse. His room is filled with junk such as strips of leather, tools and broken harnesses, his bed is a box filled with straw and few blankets thrown over it and his room just has one little window and a door that leads to the inside of the barn. He also had a few books to read because that is about all he could do in his spare time.
Another example of the poor treatment of Crooks is how the workers would excluded him from everything they did. This included when they would play cards and on a Saturday night whilst everyone went out on the town and had a good time he wasn't allowed to come he would have to just sit on his bed by himself and do nothing.
Also almost everyone working and even those living on the ranch would call him names like "nigger", which at the time was socially accepted, but nowadays is considered extremely offensive, racist and socially unacceptable. An example of this is when Lennie and Candy are in Crooks room, when all the other ranch workers are out on the town, Curley's wife comes in to Crooks' room and starts having a go at him, yelling at him and calling him names for no reason in particular other because he has dark skin.
I believe that all of the things the workers did to Crooks are very wrong because we are all human and all equal no matter what race we are or what colour our skin is.
Through this cultural assumption the author shows how poorly people of the time treated African-Americans and how racist they were.
The author challenges this assumption by allowing Lennie's character, who is the main character that everyone likes, to get to know Crooks whereas if he was reinforcing the assumption he would most certainly not allow this to happen.