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Functions of hair
Hair can have different functions:
- It can protect against light. It can absorb some UV radiation and heat.
- It can protect against getting cold too quickly; it can conserve warmth.
- It can protect against rain; many animals with fur specially treat it for this purpose.
- It can absorb excess moisture, which may come from sweating. The glands that produce the sweat are in the skin.
- It can protect against being seen and help hiding. The hair of certain animals has a coloring that changes with the seasons.
- 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.
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 race to race. Asians and native North Americans have the least amount of body hair, while Caucasians tend to have the most.
Hair color is passed down by parents only. Natural hair color can be given only by genes. It is impossible to have a hair color that is not 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.
Genetics and chemistry
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. Body hair is limited in women, and thicker in men.
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.[source?] 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.
History and culturebeards, like in Sikhism (it is also good to do this in Islam, but not a requirement).