The chords in G Major are:
GMaj7, Am7, Bm7, C Maj7, D7 (dominant 7), Em7, F#m7b5
If you played a ii V I in G Major, you have Am7, D7, GMaj7
Am7 - A C E G
D7 - D F# A C
GMaj7 - G B D F#
All these notes exist in this key of course, as they are built from the G Major scale. (Remember this is the same as your A dorian mode.)
However, if you were playing A blues, your basic chord structure would contain A7, D7 and E7.
A7 - A C# E G
D7 - D F# A C
E7 - E G# B D
Our blues scale in A is A C D Eb E G (no C#, F# or G#).
In the mode of A Dorian (A B C D E F# G), or G Major scale, the F# gives us the 3rd of our IV chord.
Our A Mixolydian mode (or D Major scale) would be
A B C# D E F# G, giving us the C# and F#, the 3rds for our I and IV chords.
Our A Lydian mode (or the E Major scale), would be
A B C# D# E F# G#, giving us the C#, F# and G#, the 3rds of our I IV and V chords, as well as our 'blue' D#/Eb note.
This is perhaps a convoluted explanation, but my point was to break down the scales/modes to relate them to chord tones, rather than trying to create a new scale/chord theory to compound our study. Modes are simply our major scales starting from different points in the scale.
If you practice from the A of your D Major scale, you will be practicing A Mixolydian.
If you practice from the A of your E Major scale, you will be practicing A Lydian.
If you practice from the A of your G Major scale, you will be practicing A Dorian; start the same scale from the D and you are playing D Mixolydian!
I would still prefer to hear 'play the key of x over this section'; modes seem to complicate matters.
Here's some links I've found:
http://www.petethomas.co.uk/jazz-minor-harmony.html
http://chrisjuergensen.com.hosting.domaindirect.com/modes_2.htm
Cheers,
SarinaJ :) http://www3.telus.net/public/wsalter/guitar/