The story of Tamar

In the book of Genesis, the story of Tamar seems to be an odd side note. It basically appears out of nowhere in the middle of a particularly exciting story. Joseph has just been sold into Egypt and then...who is Tamar? Why are we reading about Judah? What happens to Joseph? What is the point of this story?
To begin with, this story is a side note. A cutaway to a different scene as in a movie. This story is giving us a glimpse of what else is going on while Joseph’s life is taking place. It is a continuation of the life of Judah, one of Joseph’s disloyal brothers. One purpose of this separate narrative is to show that Judah, who started out betraying, continues down that same road. His actions in this story could possibly be consequences of his earlier behavior.
According to Robert Alter, this story actually takes place over a span of twenty years (p. 217). Even though it is so short, only a chapter, it may have begun far before Joseph was ever sold into Egypt.
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