This is a very interesting topic that you can analyse in this great novel. What makes it particularly interesting is the way in which, alongside the literal journey that the families undertake, the characters all undertake a more metaphorical journey as they venture towards a new understanding of themselves and of humanity as a whole.
You can easily discuss this through an analysis of the concept of "family" and how it changes as the novel progresses. Initially, the Joad family is shown to be united by blood and mutual commitment. However, the novel shows that the traditional concept of a family, not possessing a home or land to define its identity, becomes superceded by newer versions or definitions of the family. Consider the way in which the Joads and the Wilsons merge into one new, larger family that are equally committed to each other to try and ensure the groups survival. Note Steinbeck's comment about this phenomenon:
...twenty families became one family, the children were the children of all. The loss of home became one loss, and the golden time in the West was one dream.
The novel shows the Joad family journeying towards a changed attitude towards what makes a family, as migrants are forced to bond together in order to ensure their survival. This theme is demonstrated by Tom, who at the end of the novel, sees that "his" people are not just his family but really humanity as a whole.