I do not know what arguments you have found so far, but I can offer at least a few and hope that these are not some you already have.
First, there is a great deal of twin research that supports the idea that immutable characteristics such as race do not necessarily determine whom we become. Most of these studies involve twins who have been separated at birth, and while there can be striking personality similarities, it is clear that there are far more differences. This is evidence that we are not simply at the mercy of our genes.
Second, it is becoming increasingly clear that we are epigenetic beings. This means that while we have a particular set of genes, how they are acted upon dictates outcome more than the genes themselves. This could be the environment in which we are placed, but it could just as easily be the environment in which we place ourselves, meaning that if we deliberately surround ourselves with people who are good and intelligent, this will have a far greater effect on whom we become than our basic immutable genes.
Third, we now know through neuroscience that our brains are a collection of neurons and connections between them. We have a great deal of control over the configuration of those pathways. For example, if we think positive thoughts, those are the pathways that are easiest to travel in the brain. If we view ourselves negatively, or we decide that we have no control over our destinies, those become the more well-traveled paths in our brains. This does not mean that positive thinking can overcome every adversity out there, but it does mean that when we believe we have an impact on who we are, which is called a strong internal locus of control, that belief profoundly affects our power over our lives.
Finally, while this may or may not be considered a "main" argument, you should probably focus on examples of how people have overcome the baggage they are born with, to be happy, thriving, successful human beings. The world is full of such examples, from President Obama to people you already know.