Where does courage and heroism originate? Do they originate in the genes? Are they learned?

What were the origins of the firefighters’ responsibility to the men or women in front and behind them as they marched up the World Trade Center steps? How about the custodians who waded through flooded hallways to turn off electrical generators? Or the mother who quit her job and endured unimaginable grief to care for a dying child? Were these individuals born with this quality or was it learned?