By far the most common cause of hair loss in men is androgenetic alopecia, also referred to as “male pattern” or “common” baldness. It is caused by the effects of the male hormone dihydrotestosterone (DHT) on genetically susceptible scalp hair follicles. This sensitivity to DHT is present mainly in hair follicles that reside in the front, top, and crown of the scalp (rather than the back and sides), producing a characteristic and easily identifiable pattern.
It is frequently stated that “hair loss comes from the mother's side of the family.” The truth is that baldness can be inherited from either parent. However, recent research suggests that the reasons for hair loss and balding may be a bit more complex than originally thought. Factors on the X-chromosome have been shown to influence hair loss, making the inheritance from the maternal side of the family slightly more important than the paternal one. The identification of an androgen receptor gene (AR) on the X-chromosome helps to explain why the hair loss pattern of a man resembles his maternal grandfather more often than his father. However, this is clearly not the whole story since a direct inheritance of baldness from the father is observed as well. An autosomal (non-sex) linked gene would explain this type of transmission — but this gene has not yet been found.
DHT is formed by the action of the enzyme 5-alpha reductase on testosterone, the hormone that causes sex characteristics in men. DHT causes male hair loss by shortening the growth, or the anagen, phase of the hair cycle, causing miniaturization of the follicles, and producing progressively shorter, finer hairs. Eventually these hairs totally disappear.