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Human genetic variation , Race and genetics , and Human genetic clustering The understanding of the genetic mechanisms underlying human skin color variation is still incomplete, however genetic studies have discovered a number of genes that affect human skin color in specific populations, and have shown that this happens independently of other physical features such as eye and hair color. Different populations have different allele frequencies of these genes, and it is the combination of these allele variations that bring about the complex, continuous variation in skin coloration we can observe today in modern humans. Population and admixture studies suggest a 3-way model for the evolution of human skin color, with dark skin evolving in early hominids in sub-Saharan Africa and light skin evolving independently in Europe and East Asia after modern humans had expanded out of Africa. Dark skin All modern humans share a common ancestor who lived around , years ago in Africa. Research shows at least 10 differences in MC1R between African and chimpanzee samples and that the gene has probably undergone a strong positive selection a selective sweep in early Hominins around 1. Light skin For the most part, the evolution of light skin has followed different genetic paths in European and East Asian populations. They are thought to have originated after humans spread out of Africa but before the divergence of the European and Asian lineages around 30, years ago. Studies have found two alleles in the vicinity of ASIP are associated with skin color variation in humans. One, rs  has been identified as an indicator of skin reflectance in a forensics analysis of human phenotypes across Caucasian, African-American, South Asian, East Asian, Hispanic and Native American populations  and is about 3 times more common in non-African populations than in Africa. A variation in TYR has also been identified as a contributor. Research indicates the selection for the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20, years ago and perhaps as recently as 12, to 6, years ago. Neolithic farmers entering Europe at around the same time were intermediate, being nearly fixed for the derived SLC24A5 variant but only having the derived SLC45A2 allele in low frequencies. The SLC24A5 variant spread very rapidly throughout central and southern Europe from about 8, years ago, whereas the light skin variant of SLC45A2 spread throughout Europe after 5, years ago. Solute carrier family 24 member 5 SLC24A5 regulates calcium in melanocytes and is important in the process of melanogenesis. It has also been shown to be one of the significant components of the skin color of modern Europeans through its PheLeu rs  allele that has been directly correlated with skin color variation in mixed-race populations. East Asia[ edit ] A number of genes known to affect skin color have alleles that show signs of positive selection in East Asian populations. Tanning response[ edit ] Tanning response in humans is controlled by a variety of genes. All human babies are born pale, regardless of what their adult color will be. In humans, melanin production does not peak until after puberty. In some people, the armpits become slightly darker during puberty. The interaction of genetic, hormonal, and environmental factors on skin coloration with age is still not adequately understood, but it is known that men are at their darkest baseline skin color around the age of 30, without considering the effects of tanning. Around the same age, women experience darkening of some areas of their skin. Humans over the age of thirty experience a decrease in melanin-producing cells by about 10 to 20 percent per decade as melanocyte stem cells gradually die. The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes. The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between melanocytes and keratinocytes. Breastfeeding newborns, whose skeletons are growing, require high amounts of calcium intake from the mother's milk about 4 times more than during prenatal development ,  part of which comes from reserves in the mother's skeleton. Adequate vitamin D resources are needed to absorb calcium from the diet, and it has been shown that deficiencies of vitamin D and calcium increase the likelihood of various birth defects such as spina bifida and rickets. Natural selection has led to females with lighter skin than males in all indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infant and to maintain their own health. Men and women are not born with different skin color, they begin to diverge during puberty with the influence of sex hormones. Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Apart from blood-related conditions such as jaundice , carotenosis , or argyria , skin pigmentation disorders generally occur because the body produces either too much or too little melanin. Depigmentation Various types of albinism are a result of genetic mutations affecting different parts of the melanin production pathway. In a person with albinism, melanocytes can be entirely absent, or fail to produce melanin, or melanosomes can fail to mature and be transferred to keranocytes. The ability to produce melanin in patches around the body is a condition known as vitiligo. Albinism[ edit ] Some types of albinism affect only the skin and hair, while other types affect the skin, hair and eyes, and in rare cases only the eyes. All of them are caused by different genetic mutations. Albinism is a recessively inherited trait in humans where both pigmented parents may be carriers of the gene and pass it down to their children. Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity, skin cancer , and eye damage. Many cultures around the world have developed beliefs regarding people with albinism. Albinos are persecuted in Tanzania by witchdoctors , who use the body parts of albinos as ingredients in rituals and potions, as they are thought to possess magical power. It occurs when melanocytes die or are unable to function. The cause of vitiligo is unknown, but research suggests that it may arise from autoimmune , genetic, oxidative stress , neural, or viral causes. Melasma describes the darkening of the skin. Chloasma describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy. Solar lentigo , also known as "liver spots" or "senile freckles", refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure. Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes. The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as liver spots , these pigment problems are not connected with the liver. On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin. Exposure to sun[ edit ] See also: Sun tanning A suntanned arm showing darker skin where it has been exposed. This pattern of tanning is often called a farmer's tan. Melanin in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes direct and indirect DNA damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation. There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin, also known as IPD immediate pigment darkening. Secondly, there is an increase in production of melanin known as melanogenesis. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin. Tanning involves not just the increased melanin production in response to UV radiation but the thickening of the top layer of the epidermis, the stratum corneum. Generally, those who start out with darker skin color and more melanin have better abilities to tan. Individuals with very light skin and albinos have no ability to tan. Vitamin D deficiencies and UVR overexposure are concerns for many. It is important for these people individually to adjust their diet and lifestyle according to their skin color, the environment they live in, and the time of year. Fitzpatrick scale The following list shows the six categories of the Fitzpatrick scale in relation to the 36 categories of the older von Luschan scale:
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