Human hair color is a fascinating trait determined by a complex interplay of genetics and environmental factors. The pigmentation of hair follicles and shafts results from the presence of two types of melanin: eumelanin and pheomelanin. The amount and ratio of these pigments dictate the diverse range of hair colors observed in humans, from the darkest black to the lightest blond.
Melanin, a pigment derived from the amino acid tyrosine, is the primary determinant of hair color. Melanocytes, specialized cells located at the base of hair follicles, produce melanin and inject it into keratinocytes, the cells that form the hair shaft.
There are two main types of melanin:
The tone of hair color depends on the ratio of black or brown eumelanin to yellow or red pheomelanin. Blond hair, for example, contains only small amounts of both eumelanin and pheomelanin. More pheomelanin creates a more golden or strawberry blond color, and more eumelanin creates an ash or sandy blond color.
The full genetic basis of hair color is complex and not fully understood. However, several genes have been identified as playing key roles in determining hair color. The best-studied hair-color gene in humans is called MC1R.
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The MC1R 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.
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. 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. 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. Some alleles in people with brown-Swedish blond suppress the enzyme production, which in turn causes less production of brown pigment. Blond hair is formed when there is a presence of lower quantity of brown eumelanin with the absence of other pigments.
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. Genes like OCA2 and HERC2, which play a key role in determining eye color, can also impact melanin levels in hair, leading to a correlation between the two traits. For instance, lighter hair colors are more likely to accompany blue or green eyes, while darker hair often correlates with brown eyes. However, the genetic interplay is complex, and variations in other genes can result in unexpected combinations.
Hair color is inherited through genes passed down from parents to their offspring. Each parent contributes two alleles (gene variants) for hair color. A brunette may have two brown hair alleles or one brown allele and one blonde allele. The interaction of these alleles determines the child's hair color. It's important to note that because more than one gene is involved in hair color, a simple theory of dominant and recessive traits doesn’t quite capture the whole picture.
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If two brunette parents have a blond child, that means they had to have instructions for making blond hair hidden in their DNA. You probably know that DNA is organized into small pieces called genes, which help control how our bodies are built. Let’s pretend that there’s just one gene that controls hair color, with a ‘brown’ and a ‘blond’ allele. Seems simple enough, right? You have the blond allele, you have blond hair. But there’s a catch - you have two copies of each of your genes. You got one from each of your parents. It turns out that brown hair is dominant. That means that even if only one of your two alleles is for brown hair, your hair will be brown. You can think of recessive alleles as t-shirts, and dominant ones as jackets. If you wear one of each, only the jacket will be visible. In the same way, features created by recessive alleles only show up if there isn’t a dominant allele around. Since you have two copies of each gene, that means the only way to have a recessive feature like blond hair is for both of them to be the recessive allele. This is what both of our brown-haired parents had going on. With this analogy, you can also see how two blond parents should never have a brown-haired child. Since they’re both blond, they must each have two blond t-shirts and no brunette jacket.
Black hair or jet black hair is the darkest hair color. It has large amounts of eumelanin and is denser than other hair colors and is the commonly seen hair color in Asia and Africa due the fact that the people in these regions tend to have lower levels of tyrosinase in their bodies. Black eumelanin secretion causes the hair to turn black, which indicates that the MC1R is in the active state. Jet black hair, the darkest shade will not have a warm, neutral tone but a sheen which can seem almost blue, like the iridescence of a raven's wing; hence, sometimes referred to as raven-black. It is estimated that more than 90 percent of people in the world have brown or black hair.
Brown hair is the second most common human hair color, after black. Brown hair is characterized by higher levels of eumelanin and lower levels of pheomelanin. Chestnut hair is a hair color which is a reddish shade of brown hair. In contrast to auburn hair, the reddish shade of chestnut is darker.
Auburn hair ranges along a spectrum of light to dark red-brown shades. The chemicals which cause auburn hair are eumelanin (brown) and pheomelanin (red), with a higher proportion of red-causing pheomelanin than is found in average brown hair. It is most commonly found in individuals of Northern and Western European descent, but is extant in West and Central Asia and North Africa also.
Red hair ranges from light strawberry blond shades to titian, copper, and completely red. Red hair has the highest amounts of pheomelanin, around 67%, and usually low levels of eumelanin. At 1-2% of the west Eurasian population, it is the least common hair color in the world. It is most prominently found in the British Isles and in Udmurtia. Scotland has the highest proportion of redheads; 13 percent of the population has red hair and approximately 40 percent carry the recessive redhead gene. Red hair can also occur in Southern Europe, West Asia, North Africa and Central Asia. People with red hair suffer a lot due to a higher risk of sunburns and skin cancer, since the pheomelanin does not protect the skin from sunlight.
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Blond (sometimes blonde for women) hair ranges from pale white (platinum blond) to dark gold blond. Strawberry blond, a mixture of blond and red hair, is a much rarer type containing the most pheomelanin. Blond hair can have almost any proportion of pheomelanin and eumelanin, but has only small amounts of both. More pheomelanin creates a more golden or strawberry blond color, and more eumelanin creates an ash or sandy blond color. Blond hair is most commonly found in Northern and Northeastern Europeans and their descendants but can be found spread around most of Europe and also among West Asians and North Africans at lower frequencies. Studies in 2012 showed that naturally blond hair of Melanesians is caused by a recessive mutation in tyrosinase-related protein 1 (TYRP1).
Gray or white hair is not caused by a true gray or white pigment, but is due to a lack of pigmentation and melanin. Hair that lacks melanin pigments become white. The clear hairs appear as gray or white because of the way light is reflected from the hairs.
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.
Children born with some hair colors may find it gradually darkens as they grow. Many blond, light brown, or red haired infants experience this. Similar to when a baby’s hair texture changes, when a baby’s hair color will change can depend on a few factors. Both baby boy and girl hair can change color through their first few birthdays. Though, by the age of 5, most children will have their adult hair color. So when does a baby’s hair change color? It’s hard to say exactly when. But the factors involved include: hair color genetics, sun exposure, and nutrition (though genetics play the most significant role).
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.
Changes in hair color typically occur naturally as people age, eventually turning the hair gray and then white. This is called achromotrichia. Achromotrichia normally begins in the early to mid-twenties in men and late twenties in women. More than 60 percent of Americans have some gray hair by age 40. The age at which graying begins seems almost entirely due to genetics.
Melanin levels can vary over time, causing a person's hair color to change, and one person can have hair follicles of more than one color. Hair color is also somewhat dependent on external factors such as stress, sunlight, smoking, and eating poorly can also cause hair color to fade. It’s a bit like those little printers are breaking, or simply running out of ink! Marie Antoinette syndrome is a proposed phenomenon in which sudden whitening is caused by stress.
After birth, baby hair is now exposed to a new source of energy: the sun. While the sun directly impacts vitamin D production, it’s not straightforward why sunlight would cause your baby’s hair to darken over time.
Hair color can be changed by a chemical process. Permanent hair color means that the hair's structure has been chemically altered until it is eventually cut away. This does not mean that the synthetic color will remain permanently. During the process, the natural color is removed, one or more shades, and synthetic color has been put in its place. All pigments wash out of the cuticle. Permanent hair color gives the most flexibility because it can make hair lighter or darker as well as changing tone and color, but there are negatives. Constant (monthly or six-weekly) maintenance is essential to match new hair growing in to the rest of the hair, and to remedy fading. A one-color permanent dye creates a flat, uniform color across the whole head, which can look unnatural and harsh, especially in a fair shade.
Semi-permanent color washes out over a period of time-typically four to six weeks, so root regrowth is less noticeable. The final color of each strand is affected by its original color and porosity, so there will be subtle variations in color across the head-more natural and less harsh than a permanent dye. However, this means that gray and white hair will not dye to the same color as the rest of the head (in fact, some white hair will not absorb the color at all). A few gray and white hairs will blend in visually, but semi-permanent dye alone will not usually give the desired result where there is a lot of gray or white hair present. Semi-permanent hair color cannot lighten hair. Hair can only be lightened using chemical lighteners, such as bleach.
Some hair colors are associated with some ethnic groups because of the observed higher frequency of particular hair colors within their geographical region, e.g. straight, dark hair amongst East Asians, Southeast Asians, Polynesians, some Central Asians, and Native Americans; a large variety of dark, fair, curly, straight, wavy or bushy amongst Europeans, West Asians, some Central Asians, and North Africans; and curly, dark, and uniquely helical hair amongst Sub Saharan Africans. Bright red hair is found in some European populations, and hair turns gray, white, or "silver" with age. Hair color is one of the many physical traits influenced by genetic diversity within and across ethnic groups. Variations in genes such as MC1R, which affects melanin production, lead to the wide range of hair colors observed globally. These differences highlight how genetics and evolutionary pressures have shaped hair color across human populations.
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