Dominant 7 chords are formed by adding the 7th interval to a Major chord formed from the 5th mode of the Major Scale - the Mixolydian mode. In terms of the notes of a Major Scale, you end up with a chord with scale tones 1-3-5-b7 (in terms of a Mixolydian scale, you would view the tones as 1-3-5-7). So for a C Dominant 7 chord, the notes are C-E-G-Bb. As is the case with many jazz chords chord, the fifth interval is often left out.
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