Watch the Down The Line 2 online guitar lesson by Joe Deloro from Blues Rock Road Trip 2
Phrase 2 is a four bar Q&A phrase. Here, we go for a different sound than Phrase 1 by changing mixolydian modes to match each chord over the 4 bar dominant 7th chord progression.
Each mode follows the same formula: 184.108.40.206.5.6.b7,8. For G7 that's G,A,B,C,D,E,F,G. For A7 it's A,B,C#,D,E,F#,G#,A. And, with C7 it's C,D,E,F,G,A,Bb,C. However, notice that bars 1, 3, and 4 are enhanced with b3's in various ways. This minor/major approach to harmonizing phrases over Dominant 7th chords creates a distinctive blues-rock sound.
Chuck Berry's style was born in St. Louis but took off in Chicago at Chess Records in the 1950's. Carl Hogan's intro to Louis Jordan's, Ain't That Just Like A Woman was an early G7 influence. For more of a blues-rock sound, check out Eric Clapton's harmonized riff style after his solo on Cream's recording of Blind Joe Reynold's, Outside Woman Blues. In addition, be sure to check out the TAB variations for both phrases as well as combining them into an 8 bar periods for soloing.