You should definitely know hiragana before you start Genki. It would probably be best that you know both hiragana and katakana to a very well degree before starting the textbook but would not be absolutely required. You won't start kanji until Chapter 3. Chapters 1&2 have romaji under the hiragana/katakana characters so if you forget one you will know what it says. I would recommend that you know both of them before starting it. If you have trouble remembering them or once you are doing katakana you may get confused between a certain 4 similar looking characters, you could try a game called slime forest which got me past that confusion. so in short, try to get to know them both as much as possible before you start the book. It'll be easier that way. once you get to Chapter 3 there is no romaji anymore (which is a good thing) and you'll start having to do kanji from that point which if you're doing kanji you should definitely know your hiragana&katakana.