Hair color is determined by the amount of a pigment called melanin in hair. An abundance of one type of melanin, called eumelanin, gives people black or brown hair. An abundance of another pigment, called pheomelanin, gives people red hair.

The type and amount of melanin in hair is determined by many genes, although little is known about most of them. The best-studied hair-color gene in humans is called MC1R. This gene provides instructions for making a protein called the melanocortin 1 receptor, which is involved in the pathway that produces melanin. The melanocortin 1 receptor controls which type of melanin is produced by melanocytes. When the receptor is turned on (activated), it triggers a series of chemical reactions inside melanocytes that stimulate these cells to make eumelanin. If the receptor is not activated or is blocked, melanocytes make pheomelanin instead of eumelanin. Many other genes also help to regulate this process. Most people have two functioning copies of the MC1R gene, one inherited from each parent. These individuals have black or brown hair, because of the high amount of eumelanin. It is estimated that more than 90 percent of people in the world have brown or black hair.

Some people have variations in one copy of the MC1R gene in each cell that causes the gene to be turned off (deactivated). This type of genetic change is described as loss-of-function. For these individuals, eumelanin production is lower, while pheomelanin production is higher, so they have strawberry blond, auburn, or red hair. In an even smaller percentage of people, both copies of the MC1R gene in each cell have loss-of-function changes, and the melanin-production pathway produces only the pheomelanin pigment. The hair of these individuals is almost always very red. Even when the melanin-production pathway is making eumelanin, changes in other genes can reduce the amount of eumelanin produced. These changes lead to blond hair.

Hair color ranges across a wide spectrum of hues, from flaxen blond to coal black. Many genes other than MC1R play a role in determining shades of hair color by controlling levels of eumelanin and pheomelanin. Some of these genes, including ASIP, DTNBP1, GPR143, HPS3, KITLG, MLPH, MYO5A, MYO7A, OCA2, SLC45A2, SLC24A5, TYRP1, TYR, ERCC6, GNAS, HERC2, IRF4, OBSCN, SLC24A4, TPCN2, and MITF, are involved in the production of melanin in hair. Some of these genes are associated with gene transcription (which is the first step in protein production), DNA repair, the transport of substances (such as calcium) across cell membranes, or the structure of hair follicles. Several of these genes contribute to eye and skin color, but the exact role they play in determining hair color is unknown.

Hair color may change over time. Particularly in people of European descent, light hair color may darken as individuals grow older. For example, blond-haired children often have darker hair by the time they are teenagers. Researchers speculate that certain hair-pigment proteins are activated as children grow older, perhaps in response to hormonal changes that occur near puberty. Almost everyone’s hair will begin to turn gray as they age, although when it happens and to what extent is variable. Gray hair is partly hereditary and may vary by ethnic origin; it is also somewhat dependent on external factors such as stress. Hair becomes gray when the hair follicle loses its ability to make melanin, but exactly why that occurs is not clear.

Scientific journal articles for further reading

Branicki W, Liu F, van Duijn K, Draus-Barini J, Pośpiech E, Walsh S, Kupiec T, Wojas-Pelc A, Kayser M. Model-based prediction of human hair color using DNA variants. Hum Genet. 2011 Apr;129(4):443-54. doi: 10.1007/s00439-010-0939-8. Epub 2011 Jan 4. PubMed: 21197618. Free full-text available from PubMed Central: PMC3057002.

Commo S, Gaillard O, Thibaut S, Bernard BA. Absence of TRP-2 in melanogenic melanocytes of human hair. Pigment Cell Res. 2004 Oct;17(5):488-97. PubMed: 15357835.

Han J, Kraft P, Nan H, Guo Q, Chen C, Qureshi A, Hankinson SE, Hu FB, Duffy DL, Zhao ZZ, Martin NG, Montgomery GW, Hayward NK, Thomas G, Hoover RN, Chanock S, Hunter DJ. A genome-wide association study identifies novel alleles associated with hair color and skin pigmentation. PLoS Genet. 2008 May 16;4(5):e1000074. doi: 10.1371/journal.pgen.1000074. PubMed: 18483556. Free full-text available from PubMed Central: PMC2367449.

Ito S, Wakamatsu K. Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin. J Eur Acad Dermatol Venereol. 2011 Dec;25(12):1369-80. doi: 10.1111/j.1468-3083.2011.04278.x. PubMed: 22077870.

Liu F, Wen B, Kayser M. Colorful DNA polymorphisms in humans. Semin Cell Dev Biol. 2013 Jun-Jul;24(6-7):562-75. doi: 10.1016/j.semcdb.2013.03.013. Epub 2013 Apr 12. PubMed: 23587773. 

Schaffer JV, Bolognia JL. The melanocortin-1 receptor: red hair and beyond. Arch Dermatol. 2001 Nov;137(11):1477-85. PubMed: 11708951.

Siewierska-Górska A, Sitek A, Żądzińska E, Bartosz G, Strapagiel D. Association of five SNPs with human hair colour in the Polish population. Homo. 2017 Mar;68(2):134-144. doi: 10.1016/j.jchb.2017.02.002. Epub 2017 Feb 4. PubMed: 28242083.

Sturm RA. Molecular genetics of human pigmentation diversity. Hum Mol Genet. 2009 Apr 15;18(R1):R9-17. doi: 10.1093/hmg/ddp003. PubMed: 19297406.

Tobin DJ. Human hair pigmentation--biological aspects. Int J Cosmet Sci. 2008 Aug;30(4):233-57. doi: 10.1111/j.1468-2494.2008.00456.x. PubMed: 18713071.