The primary cause of hair loss in men
The primary cause of hair loss in men, Androgenitic Alopecia — or "male pattern baldness" occurs in men whose hair follicles are sensitive to the hormone dihydrotestosterone, or DHT. Typically, hair on the top and front of the head is sensitive to this hormone, and over time, this hair will become weaker and finer, and eventually will stop growing. However, lifelong hair follicles are found in good supply on the back and sides of the head in virtually all men, even men with advanced baldness. These hairs are genetically programmed to be resistant to DHT, and will generally last a lifetime. It's not that these hairs are not exposed to DHT; it is just that they are resistant to DHT, and therefore, survive for a lifetime.
Dr. Washenik explains the causes of hair loss and the principles of transplantation
For many years our schools have taught - quite erroneously - that hair loss is always inherited from the mother's father. Medical science now knows that baldness genes are passed down from both sides of the family. They also can skip generations and are utterly random in terms of which siblings they will affect. It is quite common for a woman to keep a full head of hair while her sister begins experiencing severe thinning in her twenties.