Since you are creating a post-apocalyptic (and if that apocalypse was either nuclear or biological in nature), the genetic proof your are looking for might be a mutation which is not currently present in the human gene pool.

Look around the animal kingdom for examples of biologically possible traits.  Perhaps something from our genetic neighborhood, like a prehensile tail, or maybe something from a little more distance, like reflective cats eyes.

When you through a little nuclear fallout or a few gene jokers into the mix, almost anything becomes possible.