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Yes I think the dominant 7th appears only in the Harmonic minor and the Melodic ascending minor, according to my calculations. This is because in the Natural Minor, and the Melodic Descending Minor there is a whole tone step from 7-8 and not a half tone step. And in the Dominant 7th we need a half tone step (or minor second) from 7-8 degree of the scale. This is because instead of a Major triad we would have a minor triad in the the first three notes of the Dominant 7th if we did not have a half step from the seventh degree to the eighth in the scale.
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