Androgenetic alopecia, or pattern baldness, is a widespread condition. While many believe hair loss is inherited solely from the mother’s side, its actual inheritance is more intricate, involving multiple genes and hormonal influences.
The X Chromosome Connection
A long-standing belief links baldness primarily to the mother’s X chromosome. Men inherit their single X chromosome from their mothers. The androgen receptor (AR) gene, significant for male pattern baldness, is on this X chromosome. Variations in the AR gene can make androgen receptors more sensitive to hormones, influencing hair loss. This direct maternal inheritance explains why a mother’s family history can strongly indicate male pattern baldness in sons.
A study of over 12,800 men found certain AR gene variants more than doubled the risk of male pattern baldness. While the AR gene is a substantial factor, it is not the only genetic determinant. Male pattern baldness is polygenic, meaning multiple genes are involved.
More Than Just One Gene
Androgenetic alopecia inheritance is more complex than a single X chromosome gene. Numerous other genes, located on non-sex chromosomes (autosomes), also contribute to hair loss susceptibility. These autosomal genes can be inherited from either parent, meaning both maternal and paternal genetic contributions play a part in an individual’s risk. Many genes interact, each having a small effect on the overall trait.
For instance, a 2017 review identified 63 genes influencing male pattern baldness, with only six on the X chromosome. More recent studies found hundreds of genetic variants across 350 genomic locations associated with hair loss. This wide array of contributing genes shows why simply looking at the maternal grandfather’s hair is an incomplete prediction. The combined effect of these multiple genes, inherited from both sides, determines an individual’s predisposition and the potential severity and onset of hair loss.
The Role of Hormones
Genetic predispositions interact with specific hormones to manifest as hair loss. Androgenetic alopecia is linked to androgens, male sex hormones present in both men and women. A key androgen is dihydrotestosterone (DHT), a more potent form of testosterone. DHT is formed when testosterone is converted by an enzyme called 5-alpha reductase.
In individuals with a genetic susceptibility, hair follicles on the scalp are sensitive to DHT. When DHT binds to androgen receptors in these follicles, it triggers miniaturization. This causes follicles to gradually shrink, producing thinner, shorter hairs. Eventually, these miniaturized follicles may stop producing hair, leading to visible thinning and baldness.
Understanding Your Risk
Predicting androgenetic alopecia risk involves the complex interplay of genes inherited from both parents and hormonal factors. Given hair loss’s polygenic nature, understanding family history on both maternal and paternal sides is beneficial. While the X-linked AR gene from the mother is a significant indicator, other genes from either parent also influence hair loss likelihood and characteristics. Severity and age of onset can vary even within the same family.
Current genetic tests can estimate a man’s lifetime risk of androgenetic alopecia. These tests look at variants on the X-chromosome AR gene and several autosomal locations. Such predictions are probabilistic; a high-risk score indicates an earlier onset on average, but does not guarantee hair loss. Lifestyle factors and other unknown genes also contribute, making individual outcomes variable.