Depression does appear to be, at least in part, inherited.
First-degree relatives (parents, siblings and children) of patients with major depression have been found to have a two to three times higher prevalence of major depression than their normal counterparts.
Twin studies, which look at how frequently pairs of twins have the same trait, also provide evidence of a genetic link. Pairs of fraternal (non-identical) twins, were found to have major depression at a rate of 20%. However, with pairs of identical twins (who share the same genetic material), the rate rose to about 50%.
Thus far, no genetic studies have identified what specific genes are associated with major depression. It is likely that major depression is a genetically complex condition involving multiple genes and possibly multiple modes of inheritance.
Moore, David P. and James W. Jefferson. Handbook of Medical Psychiatry. 2nd Ed. Philadelphia: Mosby, Inc., 2004.