Genes
Genetic hair thinning or baldness is due to the male hormone dihydrotestosterone (DHT) acting on genetically-susceptible scalp hair follicles. Testosterone converts to DHT with the aid of the enzyme 5-alpha-reductase. DHT then binds to receptors on hair follicles obstructing the blood supply to the follicle and thus preventing absorption of essential nutrients.
Scientists now know that DHT shrinks hair follicles through a process called miniaturization that eventually results in follicle death and hair loss. Without adequate nutrition reaching the follicles, the growth (Anagen) phase of the hair cycle shortens and the rest (Telogen) phase lengthens, so that with each new cycle the hair shaft becomes progressively smaller. The process of testosterone converting to DHT, which then harms hair follicles, occurs in both men and women.