Why is the human body hair not uniformly colored?

Why is the human body hair not uniformly colored?

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Most of my body hair is black however my lip hair is light brown/blonde the rest of my beard region is black. Since hair color is genetic what causes this?

The hairs you mention are also called "androgenic hairs", meaning their growth and pigmentation is influenced by androgens. These include pubic hair, the hairs on the breast and shoulders (almost exclusively for men) and the beard.

It seems, that these hair bulbs have different sensibilities (number and expression of androgen receptors) so they react differently to androgen levels. This includes balding, pigmentation, growth and so on.

These articles should be a good starting point, if you want to dive deeper into the topic:

I've heard it mentioned that gene dosage is implicated in this. Gene dosage is the number of the alleles present in the cell.

Is hair color determined by genetics?

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 determines hair color

Type and amount of melanin

Large amount of eumelanin

Moderate amount of eumelanin

Mostly pheomelanin with a little eumelanin

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.

Structure of Hair

The part of the hair that is located within the follicle is called the hair root. The root is the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.

Follicle and Root

Hair growth begins inside a follicle (Figure (PageIndex<2>):). Each hair follicle contains stem cells that can keep dividing and allow hair to grow. The stem cells can also regrow new hair after one falls out. Another structure associated with a hair follicle is a sebaceous gland that produces oily sebum, which lubricates and helps to waterproof the hair. A tiny arrector pili muscle is also attached to the follicle. When it contracts, the follicle moves and the hair in the follicle stands up.

Functions of Hair

In humans, one function of head hair is to provide insulation and help the head retain heat. Head hair also protects the skin on the head from damage by UV light. The function of hair in other locations on the body is debated. One idea is that body hair helps to keep us warm in cold weather. When the body is too cold, the arrector pili muscles contract and cause hairs to stand up, trapping a layer of warm air above the epidermis. However, this is more effective in mammals that have thick hair or fur than it is in relatively hairless human beings.

Figure (PageIndex<3>): This young child is using his eyebrows to good effect to convey his displeasure

Human hair has an important sensory function as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it moves because of a breeze or the touch of a physical object. The receptors may also provide sensory awareness of the presence of parasites on the skin. Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter. The eyebrows protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication (Figure (PageIndex<3>)). They help express emotions such as sadness, anger, surprise, and excitement.

How Did Different Hair Types Come To Be?


Even the lack of categorization for hair types is telling. Contrary to what your shampoo bottle may say, there is no real classification system for different hair types. At least not yet.

“Most mammals have straight hair. Only human hair [in African and Melanesian populations] has this tightly coiled configuration. We tend to talk about hair as straight, wavy, curly, in some cases frizzy,” Lasisi says. “But it’s as if we were trying to do genetic studies on height saying, there are short people, medium people, and tall people, now find what genes are related to that.”

In other words, before she could even attempt to answer the question of which genes control the texture and color of hair, Lasisi had to figure out a system for defining those hair textures and colors. Lasisi set about creating a classification system that she eventually hopes to publish, which relies on microscopic analysis of curl radius and measuring precise amounts of melanin in the hair. She then tried to answer the first of many questions: Whether tightly coiled African hair evolved in response to the hot environment. While that research is still ongoing, she says the results may indicate something counterintuitive—the thicker the hair, the better insulator it is from heat.

The Growth Cycle

The hair on the scalp grows about a half a millimeter a day—about 6 inches per year. Unlike in other mammals, hair growth and loss is random and not seasonal or cyclical. Hairs are always in various stages of growth and shed at any given time. There are three stages of hair growth: anagen, catagen, and telogen.

  • Stage 1: The anagen phase is the active or growth phase of the hair. Most hair is constantly growing and spends three to four years in this stage. A new hair forms and pushes the club hair up and out of the follicle. During this phase, hair grows approximately 1 centimeter every 28 days. Some people have difficulty growing their hair beyond a certain length because they have a short anagen phase. Conversely, people who have very long hair and have no trouble growing hair have a long anagen phase. The anagen phase for eyelashes, eyebrows, and leg and arm hair is also very short—about 30 to 45 days—which explains why these hairs are so much shorter than scalp hair.
  • Stage 2: The catagen phase is a transitional stage, and 3% of all hairs are in this phase at any given time. It lasts for two to three weeks. During this time, growth slows down and the outer root sheath shrinks and attaches to the root of the hair, forming what is known as a club hair.
  • Stage 3: The telogen phase is the resting phase, which lasts for about three months and accounts for 10% to 15% of all hair. During this phase, the hair follicle is at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, dry, white material at the root. The body sheds approximately 50 to 100 scalp hairs a day.

While many of us make periodic trips to the barber, most nonhuman mammals always appear in perfect trim without a barber. The reason for this is that hair grows in a cyclic manner. A relatively long period of growth (that varies with the type and location of the hair) is followed by a short period of rest after which the hair is released from the follicle, and a new growth cycle begins forming a new hair (see The Growth of Human Hair below). Thus the length of the growth cycle determines the length of the hair.

If hair grew longer and longer without being released from the follicle, it would be disastrous for the mammals that don’t visit a barber. Can you imagine, for example, a squirrel dashing through the branches, dragging a couple feet of hair? The Lord thinks of everything!

Hair grows about .3 mm per day (about three tenths the thickness of a dime). Within a year, our scalp and beard can produce nearly five inches (13 cm). By comparison, the longest hairs on our arm have a growth cycle of less than two months.

The growth cycle of scalp and beard hairs varies from individual to individual but can be several years. A Vietnamese man was reported to have the longest scalp hair, which measured over 20 ft. (6 m) long. According to a BBC News report in June 2004, he claimed not to have cut his hair in more than 30 years.

We stand in awe of Christ our Creator, who has lavished such exquisite design and complexity on even the hairs of our body. We are greatly comforted by Christ our Protector, who has numbered the very hairs of our head and will not permit one hair to be harmed if it is not His will. And finally, we are eternally grateful for the amazing grace of Christ our Savior who allowed His own hairs to be plucked from His cheeks as He endured taunting, torment, and death for our sins.

The Growth of the Human Hair

Hair grows from tube-like depressions in the skin called hair follicles. The hair shaft is formed from living cells deep in the follicle. These fragile living cells subsequently die to form the remarkably strong fiber we call a hair. The same follicle is capable of producing more than one type of hair during the course of our lives.

Hair grows from the bottom of the follicle at a rate of 3 tenths the thickness of a dime per day.

Hair grows to a certain length specific to each follicle then stops growing for a short period of time.

When growth begins again in the hair follicle, the resting hair is released from the follicle and a new hair is produced.

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Why Don’t Humans Have as Much Hair as Other Primates?

Although there is no definitive account of why we lost our body hair when every other primate is covered with the stuff, there are a handful of compelling theories that may explain our hairless condition.

One early explanation for why we, alone, are the only relatively “naked” apes was the aquatic ape hypothesis. First floated (pun intended) by a pathologist, Max Westenhöfer, in 1942, the idea became popular in the 1960s after it was adopted by marine biologist Alister Hardy, writer Elaine Morgan and zoologist Desmond Morris.

Essentially, the aquatic ape theory holds that during a brief period in our evolution our predecessors enjoyed a semi-aquatic existence (i.e., they lived near water and spent a great deal of time swimming, wading and diving for food). In support of the theory, they claim that we shed our hair (which would only be a drag in the water) and added, like other marine mammals, a layer of body fat. Compelling in its simplicity, the theory has been largely discredited, mostly because there is no evidence (such as in fossil record) to support it.

However, the study of human and lice genetics has produced evidence that, for whatever reason, our ancestor Homo erectus lost its hair while living in the African savanna about one million years ago. Given the location and climate, this has led some evolutionary biologists to opine that while “ running around and sweating ” in that hot climate, Homo erectus shed its heavy body hair in order to promote cooling by facilitating perspiration.

This theory has some holes, though, including that many species of monkeys that live in savannas today are very hairy, as well as the fact that, while less hair during the day would help keep a body cool, at night, it would be much more difficult to stay warm. Also detracting from this hypothesis is the fact that our nearest relative, the chimpanzee, also has less hair than it should for its size (including very little on its head), but rather than living on the hot savanna, it resides in cooler jungles.

A third popular theory is that we shed our thick hair making our bodies a less attractive home to creepy crawlers who like to feast on our blood (think lice, ticks and fleas) and spread disease. Beyond disease prevention, which is seemingly a great natural selection tool, over time, bare skin would also signal to potential partners that we have fewer parasites, which makes us more likely to be healthy, and therefore a better mate. Under this theory, bare skin was selected until it became the norm.

Another interesting hypothesis has to do with our relatively long childhood, in which we retain certain juvenile traits well past the age when other apes would have matured under this theory, it’s thought that we simply never lose the juvenile hairlessness trait. Notably, the second-least hairy ape, the chimpanzee, like us matures slowly with females not reaching reproductive age until around age 13.

A fifth theory that is receiving more attention lately proposes that we lost our hair facilitating better communication – signaling by facets of our skin and expressions on our faces. As anthropologist Barbara King described it, “ we humans possess a whole skin canvas .” Unlike many mammals, which can only see a limited range of colors such as blue, yellow and sometimes green, humans can see a much wider array this is because humans have an extra cone in our retinas (trichromatic as compared to dichromatic) that allow us to also perceive colors in the red-green zone. Altogether, our third cone allows us to distinguish the pink of a blush, the yellow of jaundice and the purple of a bruise, all of which provide an evolutionary advantage.

Interestingly, other Old World primates also have trichromatic color vision, and, although not to the same extent, less hair, particularly on their faces, when compared with mammals and New World primates who are monochromatic or dichromatic.

Did you know?

When we lost our hair it meant that nothing was protecting our skin against the sun. We think that we lost our hair at the same time as when people started getting darker skin (because darker skin protects you from the sun better than paler skin).

One person who studies genes (the instructions that your body gets from your parents) is geneticist Alan Rogers. He estimated that the gene (or instruction) that makes dark skin is just over 1 million years old. So we think this is when our ancestors were also losing their fur.

I hope you aren’t disappointed that we can’t give you a simple answer. On the other hand, we still have an interesting mystery to solve.

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