Peter-- I'll try to give you an overall explanation, and hope that it clarifies your questions--

A Scale is a pattern of whole steps and half steps that cover the span of an octave--generally, it includes seven different pitches, the lowest pitch and the highest being the same--

Each of the scales that we have been talking about(Major, Minor, Dorian Mode...etc) has it's own, distinct, order of whole steps and half steps--

Our modern, chromatic, system divides the octave up into 12 equally spaced half steps--this allows you to begin any scale (any pattern of whole steps and half steps) on any note, and apart from the difference in pitch, the relationships will sound the same--

(Note:The older modal system only divided the octave into seven parts, some of which were whole steps, and some of which were half steps--you couldn't change a Dorian mode into a major scale, because they didn't have a pitch in between the F and the G--)

(W=Whole tone, H=Half tone, WH=Whole tone plus a half tone)

If you start on G and use the major scale pattern, W-W-H-W-W-W-H, you will have a G major scale (G-A-B-C-D-E-F#-G) If you use a Dorian mode pattern, W-H-W-W-W-H-D, you will have a G Dorian mode (G-A-Bb-C-D-E-F-G)--

You may even create a scale like my Hijaz Makam (The scale used in "Miserlou") which is H-WH-H-W-H-WH-H, (G-Ab-B-C-D-Eb-F#-G)

Even though you can change a C major scale to a Lydian mode by changing the F to an F#, To change a G major scale--you must raise the fourth step in the scale, which is C, to C#(A G major scale actually has an F# in it)--

Also, don't confuse the G Dorian mode (which simply is a Dorian mode that starts on G) with the Dorian mode relative to the key of G major (which starts on A)

Basically, you have to remember that the sharps and flats are a way of adjusting the pitches to give you the intervals that you need for the scale you want--

If you want a Dorian mode that relates to the key of, say, A--you must start on the second step of the A major scale, and use only the notes that are in the A scale--which are: A-B-C#-D-E-F#-G#-A, the mode would be B-C#-D-E-F#-G#-A-B--

It is a mistake to try to try to establish an overarching relationship between the modes--the modes only exist if you use fairly narrow rules to define the place that melodic phrases end--other than this, since the notes are all the same, and you are playing over the same chords, there is no difference between playing in Phrygian mode or an Ionian mode--