DHT is present in sebum, the scalp and the blood. It is transported to the scalp and can be made in the scalp because the enzymes (called 5-alpha reductase) are present in the scalp and in sebum. Elevated scalp sebum levels may be a sign of high scalp DHT levels.
DHT and weakened scalp circulation are the two biggest causes of hair loss that can be inherited genetically, but other causes can also arguably be inherited such as a predisposition to dealing with stress badly.
Arguably all of the causes of hair loss can in some way be attributed to the genes of the sufferer. So whatever we do to treat the hair loss, we must focus on eliminating the causes and promoting factors that help the hair grow.