Steinbeck is a scientific, biological writer: his philosophy of human life is Darwinian (survival of the fittest) and descended from Pragmatism, which holds that man is, more or less, an animal and that the universe is cruel. This seemed appropriate in the 1930s and 1940s, as survival and cruelty were rampant in society across the globe (famine, drought, World War, Holocaust, genocide). Perhaps the worst 20 years in human history, these decades seemed like the last days of man, and they revealed that man had indeed moved backward socially, spiritually, and morally (to that of an animal).
Specifically, the dog in Chapter 8 is associated with Tom. There's mention of how the dog's hair "hackles," or raises up when it feels threatened, as if to fight. Ma Joad is worried that Tom will turn mean, like a dog that is bred to fight. She fears that he has been conditioned to fight in prison. She asks if the world has turned him "mean-mad" like other prisoners (Purty Boy Floyd).
Throughout the novel, Steinbeck has Tom balancing between flight and fight, and we sense that he could lash out at any time at those who exploit him and his family. Toward the end of the book he commits the same act that landed him in prison: he kills a man (the one who killed Jim Casey). So, Ma Joad's suspicions are confirmed: life in the 1930s does indeed turn the common man mean-mad.