Have you ever noticed how certain characteristics "run in the family"? Maybe it's depression. For example, a family's grandmother, mother and son all showing similar symptoms of a mood disorder. How about sleep problems? Father, father's mother, father's brother and father's children. Alcoholism, ADHD, early age heart disease and stroke?
Once you start thinking it through, it makes sense that there must be some genetic link involved in these cases. And you'd be right -- there most certainly is.