Except for identical twins, no two people in the world have the exact same genetic makeup. Research on the heredity of depression within families shows that some individuals are more likely to develop the illness than others. 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. You would also have a higher chance of developing bipolar disorder. Because close relatives of those with clinical depression have such a vulnerability to developing the condition themselves strongly suggests that it can be an inherited illness.
Bipolar disorder has a strong genetic influence.
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. Twin Studies Much of what we know about the genetic influence of clinical depression is based upon research that has been done with identical twins. Identical twins are very helpful to researchers since they both have the exact same genetic code. Because both twins become depressed at such a high rate, the implication is that there is a strong genetic influence. If it happened that when one twin becomes clinically depressed the other always develops depression, then clinical depression would likely be entirely genetic.