Editing Human variability (section)

==Sources of human variability==
[[File:Marian and Vivian Brown.jpg|thumb|[[Twin|Identical twins]] share identical genes.  They are often studied to see how environmental factors impact human variability, for example, height difference.   ]]

Human variability is attributed to a combination of environmental and genetic sources including:  {{columns-list|colwidth=30em|
*Environmental sources
**Prenatal environment
**[[Nutrition]] and [[malnutrition]]
**[[Quality of life]] and [[health care]]
**[[Pollution]] and [[toxin]] exposure and other stressors
**[[Education]]
**[[Culture]]
**[[Climate]]
**[[Family|Family environment and upbringing]] (especially before age 5)
***[[Child abuse|Child abuse and neglect]]
**[[Accidents]]
***Accidental, [[industrial injury|industrial]] or intentional injury, [[mutilation]], or change of the body
*Genetic sources
**[[Mutation]]s
***Gene mutation
***[[Chromosomal mutation]]
***External influences
**[[Sex]]ual reproduction
***[[Genetic recombination|Recombination]]
****[[Blood types]]/[[immune]] types
****[[Allele|Allelic differences]]
***[[Mate selection]]
***Reproductive capabilities
**[[Epigenetics]]
**[[Gene flow]]
}}

[[File:Unlabeled Renatto Luschan Skin color map.png|thumb|366x366px|A skin color map of the world from data collected on native populations prior to 1940, based on the [[Von Luschan's chromatic scale|von Luschan chromatic scale]]]]
For the genetic variables listed above, few of the traits characterizing human variability are controlled by simple [[Mendelian inheritance]].  Most are [[polygenic]] or are determined by a complex combination of genetics and environment.<ref>{{Cite web|url=http://www.linfo.org/human_variability.html|title=Human variation -- an introduction by The Linux Information Project (LINFO)|website=www.linfo.org|access-date=2016-11-14}}</ref>

Many genetic differences ([[polymorphism (biology)|polymorphisms]]) have little effect on health or reproductive success but help to distinguish one population from another.  It is helpful for researchers in the field of [[population genetics]] to study ancient migrations and relationships between population groups.

===Environmental factors===
====Climate and disease====
Other important factors of environmental factors include climate and disease. Climate has effects on determining what kinds of human variation are more adaptable to survive without much restrictions and hardships. For example, people who live in a climate where there is a lot of exposure to sunlight have a darker color of skin tone. Evolution has caused production of folate ([[folic acid]]) from [[Ultraviolet|UV]] radiation, thus giving them darker skin tone with more melanin to make sure child development is smooth and successful. Conversely, people who live farther away from the equator have a lighter skin tone. This is due to a need for an increased exposure and absorbance of sunlight to make sure the body can produce enough [[vitamin D]] for survival.<ref>{{Cite web|url=http://humanorigins.si.edu/evidence/genetics/human-skin-color-variation/modern-human-diversity-skin-color|title=Human Skin Color Variation {{!}} The Smithsonian Institution's Human Origins Program|website=humanorigins.si.edu|access-date=2016-11-16|date=2012-06-20|archive-date=21 November 2016|archive-url=https://web.archive.org/web/20161121012901/http://humanorigins.si.edu/evidence/genetics/human-skin-color-variation/modern-human-diversity-skin-color|url-status=dead}}</ref>

Blackfoot disease is a disease caused by environmental pollution and causes people to have black, charcoal-like skin in the lower limbs. This is caused by [[arsenic]] pollution in water and food source.<ref>{{Cite journal|last=Tseng|first=Chin-Hsiao|date=2005-01-01|title=Blackfoot disease and arsenic: a never-ending story|journal=Journal of Environmental Science and Health, Part C|volume=23|issue=1|pages=55–74|doi=10.1081/GNC-200051860|issn=1059-0501|pmid=16291522|s2cid=24276539}}</ref> This is an example of how disease can affect human variation. Another disease that can affect human variation is syphilis, a sexual transmitted disease. [[Syphilis]] does not affect human variation until the middle stage of the disease. It then starts to grow rashes all over the body, affecting people's human variation.<ref>{{Cite web|url=https://www.cdc.gov/std/syphilis/stdfact-syphilis.htm|title=STD Facts - Syphilis|website=www.cdc.gov|access-date=2016-11-16}}</ref>

==== Nutrition ====
[[Phenotype|Phenotypic variation]] is a combination of one's genetics and their surrounding environment, with no interaction or mutual influence between the two. This means that a significant portion of human variability can be controlled by human behavior. Nutrition and diet play a substantial role in determining phenotype because they are arguably the most controllable forms of environmental factors that create epigenetic changes. This is because they can be changed or altered relatively easily as opposed to other environmental factors like location. 

If people are reluctant to changing their diets, consuming harmful foods can have chronic negative effects on variability. One such instance of this occurs when eating certain chemicals through one's diet or consuming carcinogens, which can have adverse effects on individual phenotype. For example, [[Bisphenol A]] (BPA) is a known [[endocrine disruptor]] that mimics the hormone [[estradiol]] and can be found in various plastic products.<ref>{{Cite web|url=https://www.niehs.nih.gov/health/topics/agents/endocrine/|title=Endocrine Disruptors|website=www.niehs.nih.gov|access-date=2016-11-16}}</ref> BPA seeps into food or drinks when the plastic containing it is heated up and begins to melt. When these contaminated substances are consumed, especially often and over long periods of time, one's risk of diabetes and cardiovascular disease increases. BPA also has the potential to alter "physiological weight control patterns."<ref>{{Cite journal|last1=Bertoli|first1=Simona|last2=Leone|first2=Alessandro|last3=Battezzati|first3=Alberto|date=2015-07-01|title=Human Bisphenol A Exposure and the "Diabesity Phenotype"|journal=Dose-Response|language=en|volume=13|issue=3|pages=1559325815599173|doi=10.1177/1559325815599173|issn=1559-3258|pmc=4734317|pmid=26858585}}</ref> Examples such as this demonstrate that preserving a healthy phenotype largely rests on nutritional decision-making skills.

The concept that nutrition and diet affect phenotype extends to what the mother eats during pregnancy, which can have drastic effects on the outcome of the phenotype of the child. A recent study by researchers at the MRC International Nutrition Group shows that "methylation machinery can be disrupted by nutrient deficiencies and that this can lead to disease" susceptibility in newborn babies. The reason for this is because methyl groups have the ability to silence certain genes. Increased deficiencies of various nutrients such as this have the potential to permanently change the [[epigenetics]] of the baby.<ref>{{Cite web|url=http://www.lshtm.ac.uk/newsevents/news/2014/mothers_diet.html#|title=Mother's diet modifies her child's DNA {{!}} London School of Hygiene & Tropical Medicine {{!}} LSHTM.|website=www.lshtm.ac.uk|access-date=2016-11-16}}</ref>

===Genetic factors===
Genetic variation in humans may mean any variance in [[phenotype]] which results from heritable [[allele]] expression, [[mutation]]s, and [[Epigenetics|epigenetic]] changes. While human phenotypes may seem diverse, individuals actually differ by only 1 in every 1,000 [[base pair]]s and is primarily the result of inherited genetic differences.<ref>{{Cite web|url=https://www.ashg.org/education/pdf/geneticvariation.pdf|title=Genetic Variation and Human Evolution|last=Jorde|first=Lynn|date=16 October 2003|access-date=15 November 2016|archive-date=15 November 2016|archive-url=https://web.archive.org/web/20161115023305/http://www.ashg.org/education/pdf/geneticvariation.pdf|url-status=dead}}</ref> Pure consideration of alleles is often referred to as [[Mendelian Genetics]], or more properly [[Classical genetics|Classical Genetics]], and involves the assessment of whether a given trait is [[Recessive allele|dominant or recessive]] and thus, at what rates it will be inherited.<ref>{{Cite news|url=https://www.britannica.com/science/genetics#ref937023|title=genetics|newspaper=Encyclopædia Britannica|access-date=2016-11-16}}</ref> &nbsp;The color of one's eyes was long believed to occur with a pattern of brown-eye dominance, with blue eyes being a recessive characteristic resulting from a past mutation. However, it is now understood that eye color is controlled by various genes, and thus, may not follow as distinct a pattern as previously believed. The trait is still the result of variance in genetic sequence between individuals as a result of inheritance from their parents. Common traits which may be linked to genetic patterns are earlobe attachment, hair color, and hair growth patterns.<ref>{{cite web|url=https://ghr.nlm.nih.gov/primer/traits/eyecolor|title=Is eye color determined by genetics|website=Genetics Home Reference|access-date=15 November 2016}}</ref>

In terms of [[evolution]], [[Mutation|genetic mutations]] are the origins of differences in alleles between individuals. However, mutations may also occur within a person's life-time and be passed down from parent to offspring. In some cases, mutations may result in genetic diseases, such as [[Cystic fibrosis|Cystic Fibrosis]], which is the result of a mutation to the [[CFTR (gene)|CFTR gene]] that is recessively inherited from both parents.<ref>{{Cite web|url=https://ghr.nlm.nih.gov/condition/cystic-fibrosis#inheritance|title=cystic fibrosis|last=Reference|first=Genetics Home|website=Genetics Home Reference|access-date=2016-11-16}}</ref> In other cases, mutations may be harmless or phenotypically unnoticeable. We are able to treat biological traits as manifestations of either a single loci or multiple loci, labeling said biological traits as either [[Genetic disorder#Single gene disorder|monogenic]] or [[polygenic]], respectively.<ref>{{Cite web|url=https://ghr.nlm.nih.gov/primer/mutationsanddisorders/genemutation|title=What is a genetic mutation and how do mutations occur?|website=Genetics Home Reference|publisher=National Institutes of Health|access-date=15 November 2016}}</ref> Concerning [[polygenic traits]] it may be essential to be mindful of inter-genetic interactions or [[epistasis]]. Although epistasis is a significant genetic source of biological variation, it is only additive interactions that are heritable as other epistatic interactions involve recondite inter-genetic relationships. Epistatic interactions in of themselves vary further with their dependency on the results of the mechanisms of [[Genetic recombination|recombination]] and [[Crossing over, genetic|crossing over]].

The ability of genes to be expressed may also be a source of variation between individuals and result in changes to phenotype. This may be the result of [[epigenetics]], which are founded upon an organism's [[phenotypic plasticity]], with such a plasticity even being [[Heritability|heritable]].<ref name=":0">{{Cite journal|last=Feinberg|first=Andrew|year=2007|title=Phenotypic plasticity and the epigenetics of human disease|journal=Nature|volume=447|issue=7143|pages=433–440|doi=10.1038/nature05919|pmid=17522677|bibcode=2007Natur.447..433F|s2cid=4417309}}</ref><ref>{{Cite journal|last1=Jaenisch|first1=Rudolf|last2=Bird|first2=Adrian|year=2003|title=Epigenetic regulation of gene expression: how the genome integrates intrinsic and environmental signals|journal=Nature Genetics|volume=33|issue=3s|pages=245–254|doi=10.1038/ng1089|pmid=12610534|s2cid=17270515}}</ref> [[Epigenetics]] may result from [[methylation]] of gene sequences leading to the blocking of expression or changes to [[histone]] protein structuring as a result of environmental or biological cues. Such alterations influence how genetic material is handled by the cell and to what extent certain DNA sections are expressed and compose the [[epigenome]].<ref name=":0" /> The division between what can be considered as a genetic source of biological variation and not becomes immensely arbitrary as we approach aspects of biological variation such as epigenetics. Indeed, gene specific gene expression and inheritance may be reliant on environmental &nbsp;influences.

=== Cultural factors ===
{{Expand section|date=February 2021}}
Archaeological findings such as those that indicate that the [[Middle Stone Age]] and the [[Acheulean]]<ref>{{cite news |title=Neanderthal and early modern human stone tool culture co-existed for over 100,000 years |url=https://phys.org/news/2021-03-neanderthal-early-modern-human-stone.html |access-date=18 April 2021 |work=phys.org |language=en}}</ref><ref>{{cite journal |last1=Key |first1=Alastair J. M. |last2=Jarić |first2=Ivan |last3=Roberts |first3=David L. |title=Modelling the end of the Acheulean at global and continental levels suggests widespread persistence into the Middle Palaeolithic |journal=Humanities and Social Sciences Communications |date=2 March 2021 |volume=8 |issue=1 |pages=1–12 |doi=10.1057/s41599-021-00735-8 |language=en |issn=2662-9992|doi-access=free }} [[File:CC-BY icon.svg|50px]] Available under [https://creativecommons.org/licenses/by/4.0/ CC BY 4.0].</ref> – identified as a specific 'cultural phases' of humanity with a number of characteristics – lasted substantially longer in some places or 'ended' at times over 100,000 years apart, highlight a significant spatiotemporal cultural variability in and complexity of the sociocultural history and evolution of humanity.<ref>{{cite news |title=First human culture lasted 20,000 years longer than thought |url=https://phys.org/news/2021-01-human-culture-years-longer-thought.html |access-date=13 February 2021 |work=phys.org |language=en}}</ref><ref>{{cite journal |last1=Scerri |first1=Eleanor M. L. |last2=Niang |first2=Khady |last3=Candy |first3=Ian |last4=Blinkhorn |first4=James |last5=Mills |first5=William |last6=Cerasoni |first6=Jacopo N. |last7=Bateman |first7=Mark D. |last8=Crowther |first8=Alison |last9=Groucutt |first9=Huw S. |title=Continuity of the Middle Stone Age into the Holocene |journal=Scientific Reports |date=11 January 2021 |volume=11 |issue=1 |pages=70 |doi=10.1038/s41598-020-79418-4 |pmid=33431997 |pmc=7801626 |language=en |issn=2045-2322|doi-access=free }} [[File:CC-BY icon.svg|50px]] Available under [https://creativecommons.org/licenses/by/4.0/ CC BY 4.0].</ref> In some cases cultural factors may be intertwined with genetic and environmental factors.