Editing Human variability (section)

===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.