The ESV Study Bible provides a great summary of this shift, and the upcoming chapters.
The narrative now moves from the general survey of humanity to the specific family from which Israel comes. The narrative style becomes severely matter-of-fact. The narrator devotes much more time to describing the lives of the characters: whereas chs. 1–11 covers many generations in only 11 chapters, the patriarchal history deals with only four generations in 39 chapters. It begins with Abraham and goes on to his son Isaac, and Isaac’s two sons Jacob and Esau; the final section focuses on Jacob’s sons, especially Joseph. Here the specifics of being Israel are made clear: the land, the people, the blessing, and the calling. The Sinai (or Mosaic) covenant, which the first audience for these chapters receives, will provide the setting in which Israel is to put these patriarchal promises into practice. Throughout these chapters the readers will see how God has preserved the members of his chosen family, whose calling it is to walk with him, to be the headwaters of a special people and to be the channel by which blessing comes to the entire world.