Hair

Hair is something that grows from the skin of mammals. Hair is made of keratins, which are proteins.

Animal hair is usually called fur. Sheep and goats have curly hair, called wool. Wool is used to make many products, like clothing and blankets.

Humans and some other animals have lost much of their hair through evolution, and some other mammals, such as the elephant and the whale, have almost none at all.

Hair has different functions:^[1]

• It can protect against losing body heat. This is thought to be the basic, original function of hair.^[2][3]
• It protects against UV radiation, which damages the skin.
• It can protect against rain or water. Air can be trapped in the fur, or oil can be secreted by the skin. Both these methods prevent the rain or water from making the body too cold. Aquatic mammals in cold waters usually have blubber (fat) under the skin, and almost no hair.
• Defence: hair is modified in mammals like porcupines, for protection.
• Hair colouring can perform different functions. It helps to camouflage some animals. It can also signal to others of the same species. Examples are: signalling to females for mating purposes and signalling to others for territory control. Signalling danger to other species (aposematic colouring) is also done by, for example, skunks.
• Animals can change their hair so they look bigger, or more threatening. This can also be used for mating; which is the case with lions, for example. Also, the male lions' mane also protects their neck from damage when fighting other males.

Some animals, for example certain insects and spiders also have hairs. However, these are not hair in the biological sense, but are actually bristles. The hairs found on certain plants are also not true hair, but trichomes.

In humans, hair grows mostly on the head, and the amount of body hair is different from person to person.

During puberty, both men and women experience hair growth, all over their body, especially armpit and pubic hair. However, men develop thicker and more abundant body hair, and develop beards and mustaches, with hairier arms and legs, and they generally grow hair on their chest, abdomen (belly) and back. Women in comparison, have much finer hair with a less abundant distribution.

Hair color is passed down by parents only. Natural hair color can be given only by genes. Natural hair color is passed down genetically by both mother and father. This relies on dominant and recessive genes carried by a parent. These genes may not be the color of their hair, however, many people carry genes that are recessive and do not show in their traits or features.

Dyeing hair is to change the color of hair. It consists of a chemical mixture which can change the color of hair by a chemical reaction. Many people dye their hair to hide gray or white hairs. This is because most people gain white or gray hairs as they grow older.

Two types of melanin pigment give hair its color: eumelanin and pheomelanin. Pheomelanin colors hair red. Eumelanin determines the darkness of the hair color. A low concentration of brown eumelanin results in blond hair, but more brown eumelanin will color the hair brown. High amounts of black eumelanin result in black hair, while low concentrations give gray hair. All humans have some pheomelanin in their hair.

The genetics of hair colors are not yet firmly established. According to one theory, at least two gene pairs control human hair color.

One phenotype (brown/blond) has a dominant brown allele and a recessive blond allele. A person with a brown allele will have brown hair; a person with no brown alleles will be blond. This explains why two brown-haired parents can produce a blond-haired child.

The other gene pair is a non-red/red pair, where the not-red allele is dominant and the allele for red hair is recessive. A person with two copies of the red-haired allele will have red hair, but it will be either auburn or bright reddish orange depending on whether the first gene pair gives brown or blond hair, respectively.

The two-gene model does not account for all possible shades of brown, blond, or red (for example, platinum blond versus dark blonde/light brown), nor does it explain why hair color sometimes darkens as a person ages. Several other gene pairs control the light versus dark hair color in a cumulative effect (quantitative genetics).

Hair texture is also inherited genetically. The thickness of hair, its color and its tendency to curl are all inherited. There are also genetic differences between men and women.

People have in between 90,000 to 130,000 hairs on their head. About 100-150 fall out each day (depending on thickness of hair), but they usually grow back. Some men are bald but girls and women may become bald if they lose their hair from a disease called alopecia.

Men often lose some of their hair as they grow older. This is known as baldness. Doctors call it "male pattern baldness" because hairs often fall out in similar places. It often begins by hair falling out first from the front of the head, and then from the top of the head. After a while, all that may be left is a some hair running above the ears and around the lower back of the head. Even though it is unusual for women to go bald, many women suffer from thinning hair over the top of their head as they grow old.

People have tried to find cures for hair loss for thousands of years. In an effort to get their hair back, men have tried "cures" like applying strange lotions or even having their heads packed in chicken manure.^[4] Many unproven "cures" are still marketed today. It is only in the last decade or so that treatments have been developed which do sometimes work. Some doctors do hair transplants, where they take tiny plugs of hair from areas like the back of the neck and plant them in the bald spots on the head. Some drugs have been tested and approved for sale as hair loss treatments. They encourage hair regrowth and thickening, but work better if applied before hair loss turns to baldness.

People have been interested in hair on their heads for hundreds of thousands of years. For both men and women, styling and coloring hair have been ways to look good, and get attention. Sometimes society makes rules about hair, for example by not allowing people to cut their hair or beards, like in Sikhism, Judaism and Islam ^[5]

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