It is passed down through direct blood lines....it CAN be passed but will not necessarily be passed for sure. I'll used my own family as an example.

There are a cluster of illnesses that are grouped together because of the chemicals in the brain that are involved in their function: depression, bipolar illness, schizophrenia and Tourette's. There are others but those are the ones I can think of right now.

All of these relatives were on my father's side of the family. My grandmother (father's mother) had either depression or bipolar, we didn't know for sure. Two of her brothers killed themselves so obviously they also had either depression or were bipolar. Grandma suffered from depression but did not kill herself; at age 71, she suffered a stroke and then later died. She had seven children, one was my father. Of her seven children, one killed himself and one has suffered with depression all her life. Those seven children had a total of 19 grandchildren. Out of the 19 grandchildren, one is schizophrenic and one (me) is bipolar. Out of all the great grandchildren that we know of, one (my brother's son) has bipolar and Tourette's. There might be others but we are not aware of them.

So, in summary, the gene is carried down and the latest research shows that it can be passed down and even then might not be triggered by a particular event and lie dormant within a person their entire life. In another person, some life event might trigger it. So, mental illness can be quite a mystery.

Hope this helps a little.