Genetics

A gene is a single bit of chemically encoded hereditary instruction that is located on a chromosome and represents a tiny segment of DNA. Chromosomes occur in pairs (humans have 23 pairs), and every individual inherits one set of chromosomes from each parent. The genetics of androgenetic alopecia is complicated and hair loss is thought to involve more than one gene. When several genes govern a trait, it is called polygenic. Genes that are located on the X- or Y-chromosomes are call sex-linked. Genes on the other 22 pairs of chromosomes are called autosomal. It is currently believed that the genes governing common baldness are autosomal. This means that the baldness trait can be inherited from the mother's or the father's side of the family. The commonly held notion that baldness comes only from the mother's side of the family is incorrect, although for reasons not fully understood, the predisposition inherited from an affected mother is of slightly greater importance than that inherited from an affected father. The term "dominant" means that only one gene of a pair is needed for the trait to show up in the individual. A "recessive" gene means that both genes must be present in order for the trait to be expressed. The genes involved in androgenetic alopecia are believed to be dominant.
Just because a person has the genes for baldness does not mean the trait will manifest itself. The ability of a gene to affect one's characteristics, i.e. be visible in a particular individual, is called "expressivity." Expressivity relates to a number of factors, the major ones being hormones and age, although stress and other factors may play a role. To put it simply, a man whose father and uncles are severely bald may have minimal hair loss himself because the expression of the baldness gene is limited.
None of the genes responsible for male pattern baldness has yet been identified. This suggests that any kind of genetic engineering to prevent common baldness is still many years away.

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