Editing Quantitative trait locus (section)

==History==
[[Mendelian inheritance]] was rediscovered at the beginning of the 20th century. As [[Gregor Mendel|Mendel]]'s ideas spread, geneticists began to connect Mendel's rules of inheritance of single factors to [[Darwinian evolution]]. For early geneticists, it was not immediately clear that the smooth variation in traits like body size (i.e., [[Dominance (genetics)#Incomplete dominance|incomplete dominance]]) was caused by the inheritance of single genetic factors. Although [[Charles Darwin|Darwin]] himself observed that inbred features of fancy pigeons were inherited in accordance with Mendel's laws (although Darwin did not actually know about Mendel's ideas when he made the observation), it was not obvious that these features selected by fancy pigeon breeders can similarly explain quantitative variation in nature.<ref name="literature.org">{{cite web |url=http://www.literature.org/authors/darwin-charles/the-origin-of-species/ |title=Origin of Species |access-date=24 September 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131003145126/http://www.literature.org/authors/darwin-charles/the-origin-of-species/ |archive-date=3 October 2013  }}</ref>

An early attempt by [[William Ernest Castle]] to unify the laws of Mendelian inheritance with Darwin's theory of speciation invoked the idea that species become distinct from one another as one species or the other acquires a novel Mendelian factor.<ref name="ncbi.nlm.nih.gov">{{cite journal |pmid=17752783 | doi=10.1126/science.18.456.396 | volume=18 | issue=456 | title=Mendel's Law of Heredity | year=1903 | journal=Science | pages=396–406 | author = Castle WE| bibcode=1903Sci....18..396C | s2cid=11670642 | url=https://zenodo.org/record/1447900 }}</ref> Castle's conclusion was based on the observation that novel traits that could be studied in the lab and that show Mendelian inheritance patterns reflect a large deviation from the wild type, and Castle believed that acquisition of such features is the basis of "discontinuous variation" that characterizes speciation.<ref name="ncbi.nlm.nih.gov"/> Darwin discussed the inheritance of similar mutant features but did not invoke them as a requirement of speciation.<ref name="literature.org"/> Instead Darwin used the emergence of such features in breeding populations as evidence that mutation can occur at random within breeding populations, which is a central premise of his model of selection in nature.<ref name="literature.org"/> Later in his career, Castle would refine his model for speciation to allow for small variation to contribute to speciation over time. He also was able to demonstrate this point by selectively breeding laboratory populations of rats to obtain a hooded phenotype over several generations.<ref>{{cite journal|url=http://www.genetics.org/content/36/3/254|title=Variation in the Hooded Pattern of Rats, and a New Allele of Hooded|first=W. E.|last=Castle|date=1 May 1951|journal=Genetics|volume=36|issue=3|pages=254–266|doi=10.1093/genetics/36.3.254|via=www.genetics.org|pmid=14840647|pmc=1209518}}</ref>

Castle's was perhaps the first attempt made in the scientific literature to direct evolution by artificial selection of a trait with continuous underlying variation, however the practice had previously been widely employed in the development of [[agriculture]] to obtain livestock or plants with favorable features from populations that show quantitative variation in traits like body size or grain yield.{{cn|date=July 2024}}

Castle's work was among the first to attempt to unify the recently rediscovered laws of Mendelian inheritance with Darwin's theory of evolution. Still, it would be almost thirty years until the theoretical framework for evolution of [[complex traits]] would be widely formalized.<ref name="genetics.org">{{cite journal|url=http://www.genetics.org/content/16/2/97|title=Evolution in Mendelian Populations|first=Sewall|last=Wright|date=1 March 1931|journal=Genetics|volume=16|issue=2|pages=97–159|doi=10.1093/genetics/16.2.97|via=www.genetics.org|pmid=17246615|pmc=1201091}}</ref> In an early summary of the theory of evolution of continuous variation, [[Sewall Wright]], a graduate student who trained under Castle, summarized contemporary thinking about the genetic basis of quantitative natural variation: "As genetic studies continued, ever smaller differences were found to mendelize, and any character, sufficiently investigated, turned out to be affected by many factors."<ref name="genetics.org"/> Wright and others formalized population genetics theory that had been worked out over the preceding 30 years explaining how such traits can be inherited and create stably breeding populations with unique characteristics. Quantitative trait genetics today leverages Wright's observations about the statistical relationship between genotype and phenotype in families and populations to understand how certain genetic features can affect variation in natural and derived populations.{{cn|date=July 2024}}