Khác biệt giữa bản sửa đổi của “Màu da”

'''Màu da''' loài người có thể có nhiều sắc, từ rất đậm cho đến rất nhạt gần như không màu (và ở những người này, [[da]] có nước màu trắng hồng do màu [[máu]] ẩn hiện lên trên). Màu sắc của da là do lượng sắc tố [[melanin]] trong da, nhiều thì làm da đậm đen lại, ít thì làm da nhạt đi. Nói chung [[phái nữ]] có ít melanin hơn [[phái nam]].

 

Người có tổ tiên sinh sống vùng nắng nhiều, nhiệt độ cao có da đậm đen, trong khi da người có tổ tiến sống vùng ít nắng, ôn đới trắng hơn. Tuy nhiên càng về sau này càng có sự pha trộn giữa các sắc dân, nên màu da có nhiều sắc khác nhau.

 

 

== Sắc tố Melanin và gen ==

 

 

While dark skin protects vitamin B, it can lead to a vitamin D deficiency. The advantage of light skin is that it does not block sunlight as effectively, leading to increased production of [[vitamin D|vitamin D₃]], necessary for [[calcium]] absorption and [[bone]] growth. The lighter skin of women may result from the higher calcium needs of women during [[pregnancy]] and [[lactation]].

 

==Các nghiên cứu về màu da loài người==

The tone of human skin varies from dark brown to nearly colorless, which appears pale pink due to the blood in the skin. In attempting to discover the mechanisms that have generated such a wide variation in human skin tone, Nina Jablonski and George Chaplin (2000) discovered that there is a high correlation between the tone of human skin of indigenous peoples and the average annual [[ultraviolet]] (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the skin tone (W) of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that skin tone lightness W is related to the annual UV available for skin exposure AUV according to

:<math>W = 70 - \frac{AUV}{10} </math>

In considering the tone of human skin in the long span of [[human evolution]], Jablonski and Chaplin note that there is no empirical evidence to suggest that the human ancestors six million years ago had a skin tone different from the skin tone of today's [[chimpanzee]]s&mdash;namely light-skinned under black hair. But as humans evolved to lose their body hair a parallel evolution permitted human populations to turn their base skin tone dark or light over a period of less than a thousand years to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, in the time that humans lived [[Single-origin hypothesis|only in Africa]], humans had dark skin to the extent that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed light skin, though they retained within the gene pool the capability to develop dark skin when they [[Race#Phylogenetic representations|migrated]] to areas with intense sunlight again, such as across the [[Bering Strait]] and south to the Equator. [http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf]

 

Ashley Robins (1991) has argued that the light skin of indigenous peoples living north of 40° north latitude need not arise necessarily from the natural selection pressures on those humans to synthesize enough vitamin D. Robins argues that people with dark skin can get enough vitamin D if they spend enough time in the spring and summer sun, and he hypothesizes that the fatty tissues of humans would store enough vitamin D to maintain health during the northern-most winters. [http://www.amazon.com/gp/reader/0521365147]

 

Several genes have been invoked to explain variations of skin tones in humans, including ASIP, MATP, TYR, and OCA2 [http://backintyme.com/admixture/shriver01.pdf]. A recently discovered gene, [[SLC24A5]] has been shown to account for a substantial fraction of the difference in the average of 30 or so melanin units between Europeans and Africans.

 

 

==Tham khảo==

* Harding, Rosalind M., Eugene Healy, Amanda J. Ray, Nichola S. Ellis, Niamh Flanagan, Carol Todd, Craig Dixon, Antti Sajantila, Ian J. Jackson, Mark A. Birch-Machin, and Jonathan L. Rees. 2000. "Evidence for variable selective pressures at MC1R." ''American Journal of Human Genetics'' '''66''': 1351-1361.

*Jablonski, Nina G., and George Chaplin. 2000. "The evolution of human skin coloration." ''Journal of Human Evolution'' '''39''': 57-106. ([http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf in pdf format])

*Lamason RL, Mohideen MA, Mest JR, Wong AC, Norton HL, Aros MC, Jurynec MJ, Mao X, Humphreville VR, Humbert JE, Sinha S, Moore JL, Jagadeeswaran P, Zhao W, Ning G, Makalowska I, McKeigue PM, O'donnell D, Kittles R, Parra EJ, Mangini NJ, Grunwald DJ, Shriver MD, Canfield VA, Cheng KC (2005). SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans. Science 310 (5755): 1782-6. PMID 16357253

 

* Nicholas Wade. 2003. "[http://www.freerepublic.com/focus/f-news/966532/posts Why Humans and Their Fur Parted Ways]." ''New York Times (Science Times)'' ([[August 19]]). Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time at which humans invented clothing.