Is narcolepsy inherited?

Yes. Up to 10% of narcolepsy patients have family members with the illness. Twins have an even higher chance of both having the illness.
Yes. Most patients with narcolepsy with cataplexy have a genetic marker called hla dqb1*0602. However, many people without narcolepsy also this same marker. As such, simply having the marker is not sufficient to cause the condition. The current thought is that in narcolepsy an infection triggers the immune system to attack cells in the brain which secrete a chemical (orexin) needed to keep you awake.