Manual: In a manual transmission, the problem is not so much the fluid degradation, but rather fluid contamination. This contamination occurs over time as the synchronizers, bearings and gears in the transmission wear out. The resulting metal particles then float around in the lubricant.
A lot of mental illnesses can have a genetic component, yes (eg. depression, bipolar disorder, anxiety, antisocial personality disorder, etc.).
Any damage to the brain can cause a mental disorder. The brain is the control system for the nervous system and the rest of the body. Without it the body cannot function properly. Higher rates of mood, psychotic, and substance abuse disorders have been found following traumatic brain injury (TBI).