Editing Human skin color (section)

== Melanin and genes ==
{{See also|Melanin}}
Melanin is produced by cells called [[melanocyte]]s in a process called [[melanogenesis]]. Melanin is made within small membrane–bound packages called [[melanosome]]s. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the [[keratinocyte]]s. Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes. They also regulate melanin production and replication of melanocytes.<ref name=LivCol>{{cite book|last=Jablonski|first=Nina|title=Living Color|year=2012|publisher=University of California Press|location=Berkeley, Los Angeles, London|isbn=978-0-520-25153-3}}</ref> People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin.

The genetic mechanism behind human skin color is mainly regulated by the [[enzyme]] [[tyrosinase]], which creates the color of the skin, eyes, and hair shades.<ref>{{cite journal|last=Sturm|first=R. A.|title=A golden age of human pigmentation genetics|journal=Trends in Genetics|year=2006|volume=22|issue=9|pages=464–469|doi=10.1016/j.tig.2006.06.010|pmid=16857289}}</ref><ref>{{cite journal|title=Human pigmentation genes: Identification, structure and consequences of polymorphic variation|journal=Gene|year=2001|volume=277 |issue=1–2|pages=49–62|pmid=11602344|doi=10.1016/s0378-1119(01)00694-1|last1=Sturm|first1=R. A. |last2=Teasdale |first2=R. D.|last3=Box|first3=N. F.}}</ref> Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin.<ref name="LivCol" /> Melanocytes produce two types of melanin. The most common form of [[Biological tissue|biological]] melanin is [[eumelanin]], a brown-black [[polymer]] of [[indole|dihydroxyindole]] [[carboxylic acid]]s, and their reduced forms. Most are derived from the amino acid [[tyrosine]]. Eumelanin is found in hair, [[areola]], and skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin. [[Pheomelanin]], a pink to red hue is found in particularly large quantities in [[red hair]],<ref>{{cite journal|title=Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin|date=Dec 2011|journal=J Eur Acad Dermatol Venereol|issue=12|pages=1369–1380|volume=25 |doi=10.1111/j.1468-3083.2011.04278.x|pmid=22077870|last1=Ito|first1=S.|last2=Wakamatsu|first2=K.|s2cid=5121042}}</ref> the lips, nipples, glans of the penis, and vagina.<ref>{{cite web |title=pheomelanin |website=MetaCyc |url=http://www.metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |access-date=2012-02-17 |url-status=live |archive-url=https://web.archive.org/web/20150121030624/http://www.metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |archive-date=2015-01-21}}</ref>

Both the amount and type of melanin produced is controlled by a number of [[gene]]s that operate under [[incomplete dominance]].<ref>{{cite web|url=https://www.pbs.org/race/000_About/002_04-teachers-06.htm |title=The Genetics and Evolution of Skin Color: The Case of Desiree's Baby|date=2003|access-date=14 April 2015 |website=RACE—The Power of an Illusion|publisher=Public Broadcasting Service |last=Schneider |first=Patricia |url-status=live|archive-url= https://web.archive.org/web/20150506172623/http://www.pbs.org/race/000_About/002_04-teachers-06.htm |archive-date=6 May 2015}}</ref> One copy of each of the various genes is inherited from each parent. Each gene can come in several [[allele]]s, resulting in the great variety of human skin tones. Melanin controls the amount of [[ultraviolet]] (UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.