$\mathrm{Depression\ is\ often\ \underline {hereditary.}\ \mathrm {The\ underlined\ word\ is\ a/an-}}$

The underlined word "hereditary" in the sentence "Depression is often hereditary" is an adjective. It describes a characteristic of depression, indicating that it can be passed down through generations. Adjectives typically provide more information about nouns, and in this case, "hereditary" qualifies the noun "depression" by specifying its nature as being related to inheritance.