Our genes contain codes for building protein structures in our body. Those who have inherited DNA for baldness will have slightly deviant coding which creates sensitive androgen protein receptors on the follicles. Normally, these receptor molecules help protect hair structures from the damaging effects of DHT. In patients with the genetic predisposition to hair loss however, the follicles are extremely sensitive to DHT causing hair loss at physiologic levels of the hormone.
Effects of Dihydrotestosterone on hair growth
But if their structure is weaker, they will be far less resilient. And the DHT molecules will prevent the usual absorption of nutrients from the blood stream. This will cause the follicles to miniaturize and lose their ability to grow hair as they did before. Shafts may either fall out permanently or grow much thinner.
External reading: http://www.medicalnewstoday.com/articles/68082.php