Any genetic disorders that can cause adult-onset insanity or mental instability?

If so, when do they normally start showing, and are there any other symptoms?

Yes-schizophrenia usually occurs in mid to late 20s.

Here's one:

http://en.wikipedia.org/wiki/Huntington%...

There are some mental illnesses that begin in late teen years-early twenties and others that show up later in life with traumas.

People need to learn coping skills, many just get what they know from their families, if the family is dysfunctional then they are predisposed for problems sometimes.

As far as something that is insanity/ mental instability that is passed on through genes, you may be speaking of bipolar (manic depression). It has been passed in families and it begins in teenage years or early twenties. It can skip generations and sometimes only be in one gender, it just depends on the individual.

Oh my there are so many where does one start??

Depression
Anxiety,
ADD/ADHD
Schizophrenia
autism - add/ADHD are in this realm as well as so many others, schizophrenia being one a specialist told us to look out for in our child. Turrets is another.
Bi-Polar

Gosh those are just what come to mind in what a minute. Schizophrenia was once said it sets in, in late teens. Now they are saying children can develop this.
ADD/ADHD is in the autism scale and can have a onset at 3 years of age.
Depression I have not read the latest info but is very common on these boards.

Out of what I listed the only one I personally feel needs addressed with medication is the schizophrenia. If you lived with someone with this you will agree that it is a very difficult thing to live with!!

If one even thinks things are getting out of hand see a specialist. Someone who is trained in the many aspects of mental illness's. Reading information and getting other opinions is great but one who thinks things are going wrong needs a profetional...

Yes many mental illnesses seem to be carried genetically, but they usually do not show up at birth, they are usually "set off" by life circumstances
I am a rare case I hallucinated as far back as I can remember, and due to that Bi-polar problem I have multiple-personality, OCD and other problems that were more adult onset. I am now disabled
The most common is depression - and that is openly discussed you should know most of the symptoms of those
e-mail me if you want to know about other specific ones