Hair loss resulting in thinning is known as alopecia. When it is related to hormones (androgens) and genetics, it is known as androgenetic alopecia. When androgenetic alopecia denudes an area of the scalp it is called baldness. Male pattern hair loss is characterized by a receding hairline and/or hair loss on the top and front of the head. A similar type of hair loss in women, female pattern hair loss, results in thinning hair on the mid-frontal area of the scalp and is generally less severe than occurs in males.
Male pattern hair loss is an inherited condition, caused by a genetically determined sensitivity to the effects of dihydrotestosterone, or DHT in some areas of the scalp. DHT is believed to shorten the growth, or anagen, phase of the hair cycle, from a usual duration of 3–6 years to just weeks or months. This occurs together with miniaturisation of the follicles, and progressively produces fewer and finer hairs. The production of DHT is regulated by an enzyme called 5-alpha reductase.
Several genes are involved, accounting for differing age of onset, progression, pattern and severity of hair loss in family members. The susceptibility genes are inherited from both mother and father. At this time, genetic testing for prediction of balding is unreliable.
Read more about male pattern baldness at: www.malepatternbaldness.org