What Role, If Any, Does Genetics Play in Severe Depression

What role, if any, does genetics play in severe depression?

A) There is no current evidence that genetics plays a role in depression.
B) Genetic factors seem to play a role, but only in people who become depressed later in life (after age 50 or so) .
C) Genetic factors seem to play a role, but depression probably depends on several genes, not just one.
D) Genetic factors clearly control depression, and scientists have isolated the single gene that is responsible.