Are mental illnesses biological

Are mental illnesses biological? Lot’s of people believe this realistically I don’t, I think a lot of people believe this simply because there is history of mental illness in the family so therefore supposedly there is a higher likelihood of you having it because of biological defects. I don’t realistically believe that, I think it just has to do with overall upbringing, your great, great grandpa was a horrible person he was rude 24 hours a day, he never showed your grandpa any actual real love or respect and therefore later on in life when your grandpa got older he was diagnosed with clinical depression. Realistically your great grandpa had clinical depression as well, and that’s because of all the negativity brought into his life he just reflected that negativity back at your grandpa so now he has clinical depression. Now your grandpa has your dad and he’s incredibly negative with him pretty much reflecting the exact energy that his dad gave him, so now your dad starts to develop symptoms of depression, and it seems like various other mental disorders start to arise as well.