This is a great little trick you should know. Play a minor7b5 arpeggio, one and a half whole-steps below the minor chord you are playing over, and you get the Dorian sound. That's 3 frets. Pretty cool? To do the same over an A minor chord, play the F#m7b5 arpeggio. To play over a D minor chord, play Bm7b5 - do you get the idea?