Ah, Determinism Vs Freewill.
Let's look at this on a universal scale. If you replicated the universe atom by atom exactly the same right down to nuclear spins and energy states, then left the two alone for any amount of time, Determinism suggests that both universe would still be exactly the same as each other no matter how much they had changed, while Free Will suggests they would both have diverged down their own path's, which in turn suggests that one universe's basic laws are slightly altered in some way. Humans, as a mere construct of the universe should also follow those rules.
Determinism is a wonderful thing though because while it says the outcomes of every event are both predictable, or at least as predictable as a cluster of chaos systems can be, and fixed while still implying that as an individual my choices do matter to the whole since I don't know what their effect will be. I could skip class next, or I could go, what I do is already known because of what is going on around me, but because I don't know what has been determined it is still a decision I make.
Determined actions aren't based on genetics to any real extent in day-to-day life though. they're based on past actions, memories, knowledge, mood, emotion, and everything else. All that determinism states is that each thing will effect us the same way if experienced in the same conditions, and that because we can never get out of the same cycles our actions might as well be pre-determined, critically not that they are in fact pre-determined until we generate Psychohistory (look it up). Such a system could in theory be achieved only by generating a full map of the universe at the sub-atomic level (impossible) in a single yoctosecond (double impossible) using every law of nature, even the ones we don't know about or understand yet, (not quite impossible, but certainly extremely difficult to achieve on a universal scale) and any single change made to the physical world because of it would both be pre-determined, and break our model of the universe forcing us to remake the map again from scratch. Making the whole exercise impractical as well as impossible.
At the end of the day this debate comes down to weather you believe humans are just a construct of organic and inorganic molecules arranged in a form able to comprehend higher thinking and maintain the chemical balance needed for 'life', or if you think we are somehow special and able to exist on a plain higher than the physical world. I myself think we are purely physical constructs and thus believe in determinism.
If you want to read a nice long story loosely based around the challenges of free will Vs determinism I suggest the foundation series by Asimov. It deals with the potential ramifications of the deterministic system being fully understood, and how actions can seem like free will even if they have been predetermined. It does dive off the deep end in latter books though so be warned, it is still fundamentally sci-fi.