Do mental illnesses actually run in families?

yes...........................

No. Sometimes they walk and laugh like the Joker saying weird sht like "I've got a lovely bunch of coconuts"  Hate it.

Yes, and even if it’s not genetic, the mere fact that people are exposed to other mentally ill family members may end up affecting them environmentally.

Yes, there ARE TYPES of mental illnesses that are genetic and inherited.  These are the Major Affective Disorders such as Schizophrenia, Major Depression and Bipolar Disorder.  These illness are chronic and life long and they are caused by a biochemical imbalance within the brain that can only be controlled with daily medications.

Anyone can be effected by mental illness and the most common one is depression.  The American Medical Association has declared that depression is a disease and like any other disease, it responds well to medications and therapies.  It can also be terminal if not treated or under treated. 

Almost everything else is learned and can be unlearned.

And some developmental disorders such as mental retardation or the Autism Spectrum is genetic.

Intelligence is inherited.

All of the Personality Disorders are learned.

PS:  Don't listen to Jane.  Her religious preoccupation with sin and mental illness isn't healthy.  Believe me.

This is a highly complicated field and just as there are genetic medical illnesses, there are also genetic mental illnesses.  Google:  Major Affective Disorders

NOTE: MENTAL RETARDATION HAS NOT  BEEN A MEDICAL DIAGNOSIS IN A LONG TIME, ANY BIGOT THAT STILL USES THAT TERM IS NOT NOT KNOWLEDGEABLE AND THEIR ENTIRE ANSWER IS DISCREDITED.

SOME depression is strictly bio chemical and life long.....but some is induced by stress and can be 'cured'  depression often does not respond to meds.

autism runs in families but HAS NOT BEEN IDENTIFIED AS GENETIC.

there is evidence that there is some genetic pre disposition to personality disorders

schizophrenia is not an affective disorder

while some forms of INTELLECTUAL DISABILITY are due to genetic issues, often there is no known cause.

many do, some don't...for example, bipolar does, but PTSD is caused by a traumatic event, not genes

A Christian Psychiatrist may help you, but without knowledge of the Holy Bible, many people miss the truth in generational curses.  Mental illness of any kind is a sin condition.  The Bible says that a "curse without a cause does not come."  Proverbs 26:2KJV  Sin causes curses.  Not your sin, but the sins of your ancestors.  There is a book "A More Excellent Way" written by a pastor named Wright.    He will give you a study explaining the reasoning and telling you why.  Not only do mental illnesses come down as generational curses, but familiar spirits or evil spirits also come down from one ancestor to another family member.  Read the Book and then study the Word of God, the Holy Bible.

Children of a schizophrenic have a "tendency" to become schizophrenic.   How does one "learn" to have hallucinations?

Drug abuse increases the chance of developing schizophrenia for people who have the illness in their families. People who don't deal with stress well are more prone to depression than people who do, esp. people who have the illness in their families. 

It's both.  Family illnesses include both genetic and experiential factors.