In chapter 2, Lennie and George don't show up when they're supposed to and are informed by Candy that the stable buck "caught hell" for it. Sounds strange, eh? It's the 1930s. Crooks is black and faces intense racial discrimination, such as getting blamed when something goes wrong, even if he had nothing to do with it.
This is the previously alluded to obvious racism part.
The not so obvious allusion occurs a bit earlier in the chapter. George is miffed after finding a can of lice killer next to his new bunk. He's informed that the previous occupant was Whitey the Blacksmith. The disinfectant kills "greybacks," a slang term for lice. Whitey the Blacksmith left the ranch. Greybacks (what you get when you combine black and white backs, symbolically) are eliminated with a nice can of disinfectant. On this ranch, black and white just doesn't mix. Just ask Crooks.
A deeper understanding of the novel can be had by reading it as an allegory. For more on themes in the novel, check out the Of Mice and Men Study Guide.