Ionian/Mixolydian Gapped Scales and Hybrids
The Mixolydian scale is the same as the Ionian scale except that its seventh note is flattened by a semitone. To convert an Ionian to a Mixolydian key signature add a flat to, or remove a sharp from, it. Likewise, to convert a Mixolydian to an Ionian key signature, remove a flat from, or add a sharp to, it. Many Celtic, Anglo-American and English folk melodies are Ionian/Mixolydian hybrids. There may be a seventh that is sometimes flattened and sometimes not. Or the tune may be hexatonic, with no seventh at all. The Ionian and Mixolydian scales are thus quite similar to each other, and there are quite a few Ionian/Mixolydian gapped scales and quite a few Ionian/Mixolydian hybrids. Here are some examples.
08 Little Sir Hugh (Cecil J. Sharp, 1916, One Hundred English Folk Songs)
Link: Little Sir Hugh
Tune Analysis: G Ionian key signature but no F (7th). Hexatonic G Ionian/G Mixolydian gapped scale, Plagal.