Hereditary baldness, also known as androgenetic alopecia or AGA, is characterized by premature hair loss in men and women. A January 1995 article in the "The American Journal of Medicine" reports that the prevalence of hereditary baldness is between 23 and 87 percent and it increases with age. The incidence of this disorder is greater in men, but it also can occur in women. Because the presentation of the disorder is different between men, who commonly experience a receding hairline, and women, who typically experience more of diffuse hair loss, it is likely underreported in women. It also is uncommon in women who are premenopausal.
Sex-Linked Trait
A person has 46 chromosomes, two of which are sex chromosomes which determine male or female sex. Two of the genes that have been linked to hereditary baldness are on the X chromosome. A male has one X chromosome and one Y chromosome and a female has two X chromosomes. A mother who has the gene can pass it on to her sons and daughters. A father can only pass the gene to his daughters. Therefore the appearance of male baldness is passed from the maternal grandfather to grandsons. Because in families this is not always observed as the case, other genes that are not sex-linked may also contribute to hereditary baldness.
Androgen Receptor Gene
Androgens are a group of hormones that include testosterone that maintains male sex characteristics. The presence of androgens is required for baldness to manifest--males who are castrated prior to puberty do not go bald and the progression of baldness stops in males who are castrated. Androgens interact with a specific protein, called the androgen receptor on cells in many tissues, including hair follicles. A study in the July 2005 issue of the "American Journal of Human Genetics" reports that a variant in the androgen receptor is linked to male hereditary baldness. In contrast, a study published in the March 2009 issue of the "International Journal of Dermatology" did not find any correlation between the variant in the androgen receptor and the occurrence of androgenetic alopecia in women.
EDA2R
A study in the April 2008 issue of the "Journal of Investigative Dermatology" reports a second gene associated with hereditary baldness. The gene is known as EDA2R and it is also on the X chromosome, localized near the androgen receptor gene. The function of this gene is not well clarified.
Other Genes
Although only two genes have been associated with male hereditary baldness the disorder is thought to involve multiple genes. A study in the March 2008 issue of "The American Journal of Human Genetics" reports that several other gene regions are linked to hereditary baldness. These gene regions were found on the non-sex linked chromosomes 3, 11, 18 and 19. The strongest linkage between male hereditary baldness was with the region on chromosome 3.
References
- "The American Journal of Medicine"; Androgenetic Alopecia: An Autosomal Dominant Disorder; W.F. Bergfeld et al.; January 1995
- "Journal of Investigative Dermatology"; EDA2R Is Associated with Androgenetic Alopecia; Dionigio Antonio Prodi et al.; April 2008
- "The American Journal of Human Genetics"; Genome-wide Scan and Fine-Mapping Linkage Study of Androgenetic Alopecia Reveals a Locus on Chromosome 3q26; Alex M. Hillmer et al.; March 2008
- "American Journal of Human Genetics"; Genetic Variation in the Human Androgen Receptor Gene is the Major Determinant of Common Early-onset Androgenetic Alopecia; A.M. Hilmer et al.; July 2005
- "International Journal of Dermatology"; Evaluation of Androgen Receptor Gene as a Candidate Gene in Female Androgenetic Alopecia; M.H. El-Samahy et al.; March 2009
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