Voice Types according to TV Tropes:
Contraltos are the lowest female singers commonly heard. Unlike their male counterparts (the basses), however, roles for altos are a bit more common, especially in rock musicals. In more traditional pieces, altos are frequently middle-aged leading women, though some of those are mezzos as well. The only role demographic altos have a firm hold over is for women over sixty - which in most shows means grandmothers. In the rare case that there is a female villain in a show, she will probably be an alto.