Baldness

The average human head has about 100,000 hair follicles. Each follicle will grow an average of about 20 individual hairs in a person's lifetime. Average hair loss is about 100 strands a day.
Incidence of pattern baldness varies from population to population based on genetic background. Environmental factors do not seem to affect this type of baldness greatly. One large scale study in Maryborough, Victoria, Australia showed the prevalence of mid-frontal hair loss increases with age and affects 57% of women and 73.5% of men aged 80 and over. According to Medem Medical Library's website, male pattern baldness affects roughly 40 million men in the United States. Approximately 25 percent of men begin balding by age 30; two-thirds begin balding by age 60.
Male pattern is characterized by hair receding from the lateral sides of the forehead, known as "receding hairline". Receding hairlines are usually seen in males above the ages of 25 but can be seen as early as mid-teens.
An additional bald patch may develop on top (vertex). The trigger for this type of baldness (called androgenetic alopecia) is DHT, a powerful sex hormone, body, and facial hair growth promoter that can adversely affect the hair on the head as well as the prostate.
The mechanism by which DHT accomplishes this is not yet understood. In genetically-prone scalps, DHT initiates a process of follicular miniaturization. Through the process of follicular miniaturization, hair shaft width is progressively decreased until scalp hair resembles fragile vellus hair or "peach fuzz" or else becomes non-existent. Onset of hair loss sometimes begins as early as end of puberty, and is mostly genetically determined. Male pattern baldness is classified on the Hamilton-Norwood scale I-VII.
It was previously believed that baldness was inherited from the maternal grandfather. While there is some basis for this belief, both parents contribute to their offspring's likelihood of hair loss. Most likely, inheritance is technically "autosomal dominant with mixed penetrance".