Editing Quantitative trait locus (section)

===Multifactorial traits in general===
Traits controlled both by the environment and by genetic factors are called multifactorial.
Usually, multifactorial traits outside of illness result in what we see as '''continuous characteristics''' in organisms, especially human organisms such as: height,<ref name="Tissot" /> skin color, and body mass.<ref name="MedicineNet">{{cite web  | title = Definition of Multifactorial inheritance  | work = MedicineNet.com MedTerms Dictionary  | publisher = MedicineNet.com  | url = http://www.medterms.com/script/main/art.asp?articlekey=4453  | access-date = 6 January 2007  | archive-date = 17 December 2013  | archive-url = https://web.archive.org/web/20131217193339/http://www.medterms.com/script/main/art.asp?articlekey=4453  | url-status = dead  }}</ref> All of these phenotypes are complicated by a great deal of give-and-take between genes and environmental effects.<ref name="Tissot" /> The continuous distribution of traits such as height and skin color described above, reflects the action of genes that do not manifest typical patterns of dominance and recessiveness. Instead the contributions of each involved locus are thought to be additive. Writers have distinguished this kind of inheritance as ''polygenic'', or ''quantitative inheritance''.<ref name="Turnpenny">
{{cite book  | last = Turnpenny  | first = Peter  | title = Emery's Elements of Medical Genetics  | edition = 12th  | chapter = Chapter 9  | publisher = Elsevier  | date = 2004  | chapter-url = http://www.fleshandbones.com/readingroom/viewchapter.cfm?ID=1041  | chapter-format = PDF  | access-date = 6 January 2007 }}
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Thus, due to the nature of polygenic traits, inheritance will not follow the same pattern as a simple [[monohybrid cross|monohybrid]] or [[dihybrid cross]].<ref name="Clinical Genetics" /> Polygenic inheritance can be explained as Mendelian inheritance at many loci,<ref name="Tissot" /> resulting in a trait which is [[normal distribution|normally-distributed]]. If ''n'' is the number of involved loci, then the coefficients of the [[binomial distribution|binomial expansion]] of (''a'' + ''b'')<sup>''2n''</sup> will give the frequency of distribution of all ''n'' allele [[combination]]s. For sufficiently high values of ''n'', this binomial distribution will begin to resemble a normal distribution. From this viewpoint, a disease state will become apparent at one of the tails of the distribution, past some threshold value. Disease states of increasing severity will be expected the further one goes past the threshold and away from the [[statistical mean|mean]].<ref name="Turnpenny" />