Some mental disorders tend to be fairly heritable while others seem to be developmental. Among those that are categorized as highly heritable is bipolar disorder. A mood disorder causing the patient severe depression and manic episodes, bipolar disorder is very difficult to treat and to live with.
No parent would want their child to be at a heritable risk of this, and yet the chances of heritability are heartbreaking. If one parent has been diagnosed with bipolar, the chances of any of their children inheriting the disorder is around 15%-30%. If both parents have bipolar disorder, the odds jump to 50%-75%. The 2003 study that revealed these results showed why bipolar disorder tends to run in families; it is carried in our genes, so to speak. A twins study showed that if one identical twin had bipolar disorder, the other twin had an 85% chance of having it as well. Even if an adult manages to control their own bipolar disorder, the odds are against them when they have children.