Can depression be inherited?
Depression is highly familial, which means it can run in families and may be transmitted by genes. When a parent has depression, a child faces about a two- to three-times increased risk of becoming depressed compared to a child without a depressed parent. If the parent developed the mental illness before age 20, the child’s risk rises even higher. Depression in second-degree relatives, such as grandparents, aunts and uncles, or cousins, carries a much lower familial risk.
"Risk" doesn't mean that an outcome is inevitable. Not all kids or adults who have a depressed parent -- or even have both parents that are depressed -- necessarily get depression themselves. Even if a person's identical twin, who has the exact same genetic makeup, had depression, that person's risk for developing depression isn't much more than 50%. So other factors besides genetics must play a role.
Depression is a complex disorder in which genes, environment, and other factors such as personality or other psychological features all play a role. So far, research suggests genetic vulnerability that makes some people more likely to develop depression. However, unlike "purely" genetic diseases such as cystic fibrosis or Tay-Sachs, there probably isn't one single gene that causes depression. Rather, there are probably many different genes that each exert small effects which can add up to an overall genetic risk for depression.