thank you for your query. Depression is a mental disease which follows the gene -environment model of heritabilty (very much like diabetes type II), rather than a pure mendelian transmission (like thalassemia,sickle cell disease etc.). in diseases like thalassemia, if a person has a trait, and produces a child with another person who also has a trait, it is a very high chance that the child will have full blown thalassemia, irrespective of the environment he lives in. but in diseases like depression, presence of depression in family certainly genetically predispose an individual, but symptomatic depression only occurs if there is an element from the environment, like stress, difficult living conditions, etc. overall, there is an increased risk by 30- 70% in offsprings of depressed parents, but please remember, there is a huge role of the environment also. I would suggest you to ask your friend to discuss this matter with his fiance open terms if its bothering him. More importantly, if they decide to get married and conceive, it has to be under the supervision of her treating psychiatrist, as certain antidepressants are best avoided in pregnancy,and discussing pregnancy plans with the doctor will help in choosing safe medications. hope i ve been able to satisfy your query. please feel free to contact me for any further clarifications. best of luck.