There was recently a study of 66,000 people (half with mental illness and half without,) that found similar problems with the same gene for 5 illnesses. Autism, schizophrenia, bipolar, depression, and ADD. It is the same gene but effects people differently. This research is very new.

The population was huge.

However, at this time the research needs to be repeated and more needs to be learned on it. There is no gene testing that you can have by a doctor. If you're interested in this, I would say search for finding out how to become part of the research.

You can read about the study here:
The Surprising Link Between Depression, ADHD, Bipolar, Schizophrenia and Autism