Epigenetics explains why genes are not deterministic in and of themselves. This is why theories on mental illness that try to blame genes are always over-simplified. Environment and what you eat and the amount of stress or trauma one lives with all are part of the picture.
Hank & his clone Circus Hank explain the power of epigenetics, which studies the factors that determine how much or whether some genes are expressed in your body.
Another post on epigenetics here: The Ghost in your Genes: an intro to epigenetics