Complex sentence: The only exception is the Dorian mode of the second scale degree, where the sixth is avoided although it is a whole step above the fifth; this is because the tritone between this and the third scale step would give an unwanted dominant flavour.
Split into simpler sentences: The only exception is the Dorian mode of the second scale degree, where the sixth is avoided, though it is a whole step above the fifth. This is because the tritone between this and the third scale step would give an unwanted dominant flavour.