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.).
There are many causes of mental disorders. Your genes and family history may play a role. Your life experiences, such as stress or a history of abuse, may also matter. Biological factors can also be part of the cause. A traumatic brain injury can lead to a mental disorder.