I disagree with your statement about jazz and folk songs being sung by altos, Lady P. Most of the singers of these genres that I've hung around with favor the lower range. In fact, it makes it a little difficult for me when I participate in a workshop at a festival, as by the end of the week my voice is hoarse from singing much lower than I'm used to. And I'm only a mezzo soprano, and have sung alto parts in choirs (although I'm sometimes discouraged from singing alto because my tone is not dark enough--I sound more like a lyric soprano).