Freewill: A Reality or an Illusion?

This quarter in biology, we've been learning about biological and environmental determinants of behavior. For example, obesity and drug addiction have biological correlates. People who are obese tend to have fewer active dopamine receptors. Dopamine is released when we eat pleasant tasting food or experience something novel. Since obese people have fewer dopamine receptors, that means they have to eat MORE in order to feel just as satiated as someone who has a greater amount of active dopamine receptors. Of course obese people still "choose" to put the food in their mouth, and thus, most people would argue that this proves they have "freewill" in the matter. But the fact that their neurochemistry is abnormal cannot be overlooked. True, while they still choose to put food in their mouths, the amount of food they eat (leading to obesity) is influenced, from a biological view, by how satiated they feel. Now we obviously make choices everyday, I do not deny that at all. However, it seems that WHAT we choose to do is always influenced by some external factor(s), such as our biological predispositions, our past experiences, our knowledge, our values, beliefs, etc. Given this, to what extent is our behavior a TRUE and absolute expression of freewill? If the choices we make are always influenced by prior causes which tend to be out of our direct control, how can we really say that we have freewill? Do we FREELY choose what we "will" (or want), or do our genes and environments both play a role in shaping our desires and behavior?

I believe by the ability to go against the grain, (or our genes) our will does remain free... Because yes external factors may play a role but they dont force us to act... We still decide whether not to act upon them... On that note, Ima go knock this bank off... and plea "my father was a robber so i aint have a choice" in court....