This is a very focused, but nonetheless possible, question. It is important to know in general how genes and gene x environment interactions affect human behaviour. But it is also vital to know how one gene in particular works.
Possible responses include material from Caspi et al. 2002 which discusses how a polymorphism (variation) in the gene encoding MAOA is related to adult aggression as a delayed response to earlier childhood maltreatment. Caspi et al.2003 is a natural experiment that identifies a correlation between stress, a polymorphism on the HTT serotonin-transporter gene and depression. There are other possibilities.
An answer to this question would need to focus on explaining how the gene expression works in the brain to affect behaviour. Both genes mentioned above work in interaction with the environment to create a stronger response to lifetime events (maltreatment or stress) than is shown by those without the polymorphism. It is enough for students to learn about just one of these. It is generally agreed nowadays that gene expression does not happen in isolation, but interacts with environmental predisposing conditions. We are not just responding to our genes, but equally, we are not just responding to our environment. As ever, the picture is complex.