How does the brain inherit risk for mental illness?

Conditions such as depression, bipolar disorder, schizophrenia, and ADHD have substantial heritable components, but they are not caused by one gene alone. Instead, many genetic variants each contribute a small amount of risk. Environmental factors such as stress, trauma, sleep disruption, and substance use can also affect whether symptoms develop and how severe they become. Research using family and twin studies shows that genes matter, but they do not determine destiny.