(My reading of 'The Grapes of Wrath' was heavily influenced by the plot analysis approach explained in 'The Seven Basic Plots' by Christopher Booker. This review contains a host of (view spoiler)[spoilers.(view spoiler)[) 
 Plot form: Tragedy (or the inverse of Overcoming The Monster) 
 Central Theme: The denial of masculinity by big oppressive forces and the destructive effect this has on the family. 
 Each of the main characters in 'The Grapes of Wrath' are 'twilight' characters, or those which are neither fully good nor fully bad. Prior to the beginning of the story, each man has already displayed out of balance masculinity -- Tom spent time in prison for unintentionally killing a man, Pa injured his oldest son during birth, Uncle John caused the death of his wife by not taking her illness seriously, and Al has been sexually active with a number of women. 
 When the Dust Bowl hits and the farmers lose their farms to the banks, the farmers desire to fight back, but due to the inhuman, faceless banks, there is nothing to focus their energies in an attack -- their masculine desire to protect their property and way of life is denied. 
 When moving across the country to California, the men display masculinity through their determination to provide a better life for their families. However, upon arriving and realizing their bleak employment prospects, Connie walks away from the family never to return, leaving behind his pregnant wife. 
 While the Joad family finds temporary comfort in the government camp in Weedpatch (I've been to Weedpatch!), the men are not able to find work. It is during this period when the family is provided with comforts without the toil of their men, that Pa fades away as the patriarch of the family and Ma takes his place in leadership. From then on, she makes decisions for the family and rides up in the front seat while Pa rides in the back with Uncle John. 
 When Tom kills another man through his imbalanced masculinity, he is no longer able to provide for his family because he must go into hiding. This places an extra strain on Al, who eventually separates from the family when he gets engaged to a neighbor girl -- his masculine expression of getting engaged simultaneously weakens his ability to assist his parents who need his automobile mechanic skills. 
 While the men are denied and lose their masculinity (and it becomes twisted into the Dark Inversion) the women also experience an inversion of their femininity -- with Ma becoming the patriarch and Rose of Sharon breast-feeding an adult man to temporarily prolong his life. 
 Despite their gender role reversals and the weakening of their family (we are left with the bleak prospect of their eventual starvation), they do show a general fighting spirit to maintain their humanity. However, due to the monstrous oppressive forces of big agriculture, their desires are overcome and they are helpless to the destructive forces of nature and fate. 
 Interestingly, while 'The Grapes of Wrath' could be seen as a pro-Marxist book, it is fundamentally a pro-human freedom/pro-family/anti-oppression book. Since Marx called for the abolition of the family 'The Grapes of Wrath' could hardly promote a Marxist agenda, but perhaps Steinbeck left out that detail. (hide spoiler)] (hide spoiler)]