The natural color of human hair is determined by the presence and concentration of pigment molecules called melanin. This complex polymer is synthesized within the body and deposited into the hair shaft as it grows. The entire spectrum of human hair color, from black to bright red, results from the interplay of two distinct types of melanin. The pigment contributing to lighter and warmer shades, including red, ginger, and yellow-blond tones, is known as pheomelanin.
Pheomelanin The Pigment of Warm Tones
Pheomelanin is chemically distinct from its darker counterpart, accounting for its reddish-yellow hue. A defining feature of pheomelanin’s structure is its significant sulfur content, incorporated during synthesis. This sulfur comes from the amino acid L-cysteine, which reacts with the melanin precursor molecule to form the unique polymer. The final structure is composed of benzothiazine and benzothiazole units, responsible for reflecting light in the warmer color spectrum.
The concentration of pheomelanin dictates whether the hair appears yellow-blond or vibrant red. When the hair shaft contains a very low amount of pigment overall, but the small amount present is primarily pheomelanin, the resulting color is light yellow or golden blond. A much higher concentration of pheomelanin produces the characteristic shades of red and ginger hair. This pigment is chemically less stable than eumelanin, which contributes to the tendency of red and blond hair to lighten more easily when exposed to sunlight.
How Pigment Ratios Determine the Full Color Spectrum
The complete range of human hair color is achieved through a mixture of pheomelanin and a second pigment called eumelanin. Eumelanin is the dark, granular pigment responsible for brown and black coloration. All hair colors are a blend of these two types, and the specific shade depends entirely on their relative ratio and total concentration within the hair shaft.
Hair that is black or dark brown contains a high concentration of eumelanin and only trace amounts of pheomelanin. As the concentration of eumelanin decreases, the hair color lightens, moving through the spectrum from dark brown to light brown. Blond hair results from low concentrations of both pigments. The small amount of eumelanin present in blond hair is typically the brown subtype, not the black.
The vibrant colors of auburn and deep red hair occur when there is a significant presence of pheomelanin mixed with a moderate amount of brown eumelanin. The highest levels of pheomelanin combined with very low levels of eumelanin produce the classic red-ginger shades. The final visible hair color is a precise chemical phenotype reflecting the exact balance of these two melanin types.
The Cellular Origin of Hair Color
The production and deposition of these pigments occur in specialized cells called melanocytes, located at the base of the hair follicle. Melanocytes are found in the hair bulb, the base structure where the hair shaft begins to form.
Within each melanocyte are tiny, membrane-bound sacs known as melanosomes, which are the sites of melanin synthesis. Inside these melanosomes, biochemical reactions convert the amino acid tyrosine into either eumelanin or pheomelanin. As the hair shaft grows upward, the melanocytes transfer the loaded melanosomes into the surrounding keratinocytes, the cells that form the hair fiber.
The hair color is then “locked in” as the keratinocytes die and harden, incorporating the melanin granules into the hair shaft’s cortex. This continuous process ensures that the new hair maintains a consistent color. A disruption in this cellular transfer or synthesis process, such as with age, leads to the loss of color and the development of gray or white hair.
The Genetics That Control Pigment Production
The decision of whether a melanocyte produces more eumelanin or pheomelanin is governed by an individual’s genetic code. One primary gene controlling this switch is the Melanocortin 1 Receptor (MC1R) gene. This gene provides instructions for making a protein receptor found on the surface of the melanocyte.
When this receptor is fully functional and activated, it signals the melanocyte to prioritize the production of the dark pigment, eumelanin. Variations or mutations in the MC1R gene can reduce the receptor’s ability to respond to these signals. This reduction in function causes the melanocyte to default to producing the reddish-yellow pheomelanin.
Individuals with two copies of a non-functional MC1R variant typically exhibit red hair because their melanocytes primarily produce pheomelanin. Hair color is a polygenic trait, meaning other genes also contribute to the final shade by affecting the overall amount and distribution of pigment. These genetic instructions determine the internal chemical environment of the melanocyte, which dictates the precise ratio of the two melanin types.