Mental illnesses are known to run in families.

Allaboutdepression.com says,

If you have a parent or sibling that has had major depression, you may be 1.5 to 3 times more likely to develop the condition than those who do not have a close relative with the condition.

The link is even stronger with bipolar disorder:

Of those with bipolar disorder, approximately 50% of them have a parent with a history of clinical depression. When a mother or father has bipolar disorder, their child will have a 25% chance of developing some type of clinical depression. If both parents have bipolar disorder, the chance of their child also developing bipolar disorder is between 50% and 75%. Brothers and sisters of those with bipolar disorder may be 8 to 18 times more likely to develop bipolar disorder, and 2 to 10 times more likely to develop major depressive disorder than others with no such siblings.

However, a specific gene linked to depression has not been found — yet.

But a new study suggests that a gene could help determine why people respond better to certain antidepressants than others

The German team looked at how changes in the ABCB-1 gene affected three widely used treatments: Forest Laboratories Inc’s Celexa, Wyeth ’s Effexor and Remeron from Dutch chemicals group Akzo Nobel’s Organon pharmaceutical unit. … They found that the gene and the protein blocked Celexa and Effexor but not Remeron. The researchers do not know why but said the drugs’ different chemical makeup could be the reason.

The team added that a genetic test could be developed to help people for whom antidepressants don’t work.