I think it all depends where you go to school. A lot of my classes are separate lectures and labs (but required for my degree). I had a genetics lecture and lab as well as cell bio and lab. Most bio classes at our school are 3 hour lecture and 2 hour lab credits. I think chem/physics combo is harder than genetics/cell bio would be. Chem and physics are both math intensive courses that require studying and memorizing formulas, constants, etc. Genetics/cell bio are both a lot of material but I think are easier to memorize. And depending on what your cell bio prof. focuses on, you will see some overlap of material between cell bio and genetics.