Hi anna.253,
I'm just posting this just in case someone else has this issue as you and I have had. jeremy_v, thanks for your guidance, but you left out an important step which I consider an easter (hidden) egg to be able to shuffle all music in one genre. I have the latest update...iOS 9.2.1
1. touch your Music app, then my music
2. in the middle highlighted in red...artists, albums, songs, genres....select genres
3. your genres should be listed, if you have them organized
4. (hidden) egg...touch the thumbnail on left side of your genre, then make sure that shuffle button is selected on your mini-player after you expand it.
5. You've done it!!! To the right of the "next song arrow" is the "up next," when you touch that it'll show you the next songs in that genre.