Not all breeding projects are about specific genetic predictions like recessive and dominant traits. There's also this thing called selective breeding...which unfortunately is less commonly practiced in the ball python arena. When you find a polymorphic trait (which is likely what that back stripe is) that you like, you can work on breeding that trait into your offspring. It just doesn't happen as predictably or with as much assurance as known individual genetic traits. The more you breed high quality traits back to each other, the more likely they are to be expressed in future generations...whether that be back stripes, clean patterns, black backs, bright colors....whatever!