Editing Polymorphism (biology) (section)

== Genetics ==

=== Genetic polymorphism ===
{{Main|Gene polymorphism}}

Since all polymorphism has a genetic basis, ''genetic polymorphism'' has a particular meaning:
* Genetic polymorphism is the simultaneous occurrence in the same locality of two or more discontinuous forms in such proportions that the rarest of them cannot be maintained just by recurrent mutation or immigration, originally defined by Ford (1940).<ref name="Ford 1940" /><ref name="Ford 1965" />{{rp|11}} The later definition by Cavalli-Sforza & Bodmer (1971) is currently used: "Genetic polymorphism is the occurrence in the same population of two or more alleles at one locus, each with appreciable frequency", where the minimum frequency is typically taken as 1%.<ref name="Hedrick2011">{{cite book |last=Hedrick |first=Philip |title=Genetics of Populations |url=https://books.google.com/books?id=pBq69Luwf7UC&pg=PA104 |date=2011 |publisher=Jones & Bartlett Learning |isbn=978-0-7637-5737-3 |page=104}}</ref><ref name="Cavalli-SforzaBodmer1999">{{cite book |last1=Cavalli-Sforza |first1=Luigi Luca |last2=Bodmer |first2=Walter Fred |title=The Genetics of Human Populations |url=https://books.google.com/books?id=rdZNbApUGUsC&pg=PP118 |year=1999 |orig-date=1971 |publisher=Courier |isbn=978-0-486-40693-0 |pages=118–122}}</ref>

The definition has three parts: a) [[sympatry]]: one interbreeding population; b) discrete forms; and c) not maintained just by mutation.{{cn|date=December 2024}}

In simple words, the term polymorphism was originally used to describe variations in shape and form that distinguish normal individuals within a species from each other. Presently, geneticists use the term genetic polymorphism to describe the functionally silent differences in [[DNA]] sequence between individuals that make each human genome unique.<ref>Weinberg, Robert A. (Robert Allan), 2013 "The biology of cancer". 2nd edition, Garland Science, Taylor & Francis {{ISBN|978-0-8153-4219-9}}</ref>

Genetic polymorphism is actively and steadily maintained in populations by natural selection, in contrast to ''transient polymorphisms'' where a form is progressively replaced by another.<ref name="Begon">Begon, Townsend, Harper. 2006. ''Ecology: from individuals to ecosystems''. 4th ed, Blackwell, Oxford. {{ISBN|978-1-4051-1117-1}}</ref>{{rp|6–7}} By definition, genetic polymorphism relates to a balance or equilibrium between morphs. The mechanisms that conserve it are types of [[balancing selection]].{{cn|date=December 2024}}

=== Mechanisms of balancing selection ===
* [[Heterosis]] (or [[heterozygote advantage]]): "Heterosis: the [[Heterozygous|heterozygote]] at a [[Locus (genetics)|locus]] is fitter than either [[Homozygous|homozygote]]".<ref name="Ford 1975" /><ref name="Smith 1998" />{{rp|65}}<ref name="Ford 1965">Ford, E. B. 1965. "Heterozygous Advantage". In ''Genetic Polymorphism''. [[Boston, Massachusetts|Boston]]/London.: [[MIT Press|MIT Pr.]]/[[Faber & Faber]]</ref>
* [[Frequency dependent selection]]: The fitness of a particular phenotype is dependent on its frequency relative to other phenotypes in a given population. Example: [[prey switching]], where rare morphs of prey are actually fitter due to predators concentrating on the more frequent morphs.<ref name="Ford 1975" /><ref name="Begon" />
* Fitness varies in time and space. Fitness of a genotype may vary greatly between larval and adult stages, or between parts of a habitat range.<ref name="Ford 1965" />{{Rp|26}}
* Selection acts differently at different levels. The fitness of a genotype may depend on the fitness of other genotypes in the population: this covers many natural situations where the best thing to do (from the point of view of survival and reproduction) depends on what other members of the population are doing at the time.<ref name="Smith 1998" />{{rp|17 & ch. 7}}

=== Pleiotropism ===

Most genes have more than one effect on the [[phenotype]] of an organism ([[Pleiotropy|pleiotropism]]). Some of these effects may be visible, and others cryptic, so it is often important to look beyond the most obvious effects of a gene to identify other effects. Cases occur where a gene affects an unimportant visible characteristic, yet a change in fitness is recorded. In such cases, the gene's subsurface effects may be responsible for the change in fitness. Pleiotropism is posing continual challenges for many clinical dysmorphologists in their attempt to explain birth defects which affect one or more organ system, with only a single underlying causative agent. For many pleiotropic disorders, the connection between the genetic abnormality and its manifestations is neither apparent nor understood.<ref>{{Cite book |title = Genetics in Medicine |last = Nussbaum |first = Robert L. |publisher = Thompson & Thompson |year = 2007 |isbn = 9781416030805 |location = Canada |pages = 116, 422}}</ref>
:"If a neutral trait is pleiotropically linked to an advantageous one, it may emerge because of a process of natural selection. It was selected but this doesn't mean it is an adaptation. The reason is that, although it was selected, there was no selection for that trait."<ref>Sober E. 1984. ''The nature of selection: evolutionary theory in philosophical focus''. Chicago. p197</ref>

=== Epistasis ===

[[Epistasis]] occurs when the expression of one gene is modified by another gene. For example, gene A only shows its effect when allele B1 (at another [[Locus (genetics)|locus]]) is present, but not if it is absent. This is one of the ways in which two or more genes may combine to produce a coordinated change in more than one characteristic (for instance, in mimicry). Unlike the supergene, epistatic genes do not need to be closely [[Genetic linkage|linked]] or even on the same [[chromosome]].{{cn|date=December 2024}}

Both pleiotropism and epistasis show that a gene need not relate to a character in the simple manner that was once supposed.{{cn|date=December 2024}}

=== The origin of supergenes ===

Although a polymorphism can be controlled by [[alleles]] at a single [[Locus (genetics)|locus]] (e.g. human [[ABO]] blood groups), the more complex forms are controlled by [[supergene]]s consisting of several tightly [[linked genes]] on a single [[chromosome]]. [[Batesian mimicry]] in butterflies and [[heterostyly]] in angiosperms are good examples. There is a long-standing debate as to how this situation could have arisen, and the question is not yet resolved.

Whereas a [[gene family]] (several tightly linked genes performing similar or identical functions) arises by duplication of a single original gene, this is usually not the case with supergenes. In a supergene some of the constituent genes have quite distinct functions, so they must have come together under selection. This process might involve suppression of crossing-over, translocation of chromosome fragments and possibly occasional cistron duplication. That crossing-over can be suppressed by selection has been known for many years.<ref>{{cite journal |author1=Detlefsen J.A. |author2=Roberts E. | year = 1921 | title = Studies on crossing-over I. The effects of selection on crossover values | url = https://zenodo.org/record/1426882| journal = Journal of Experimental Zoology | volume = 32 | issue = 2| pages = 333–54 | doi = 10.1002/jez.1400320206 |bibcode=1921JEZ....32..333D }}</ref><ref>Darlington, C. D. 1956. ''Chromosome Botany'', p. 36. London: [[Allen & Unwin]].</ref>

Debate has centered round the question of whether the component genes in a super-gene could have started off on separate chromosomes, with subsequent reorganization, or if it is necessary for them to start on the same chromosome. Originally, it was held that chromosome rearrangement would play an important role.<ref>Darlington, C.D.; Mather, K. 1949.  ''The Elements of Genetics'', pp. 335–336. London: Allen & Unwin.</ref> This explanation was accepted by E. B. Ford and incorporated into his accounts of ecological genetics.<ref name="Ford 1975" />{{rp|ch. 6}}<ref name="Ford 1965" />{{rp|17–25}}

However, many believe it more likely that the genes start on the same chromosome.<ref>{{cite journal | issn = 0022-5193 | volume = 55 | issue = 2 | pages = 283–303 | last = Charlesworth | first = D |author2=B Charlesworth | title = Theoretical genetics of Batesian mimicry I. single-locus models | journal = [[Journal of Theoretical Biology]] | year = 1975 | pmid =  1207160 | doi=10.1016/s0022-5193(75)80081-6| bibcode = 1975JThBi..55..283C }}<br />{{cite journal | issn = 0022-5193 | volume = 55 | issue = 2 | pages = 305–324 | last = Charlesworth | first = D. |author2=B. Charlesworth | title = Theoretical genetics of Batesian mimicry II. Evolution of supergenes | journal = [[Journal of Theoretical Biology]] | year=1975 | pmid=1207161 | doi=10.1016/s0022-5193(75)80082-8| bibcode = 1975JThBi..55..305C }}<br />{{cite journal | issn = 0022-5193 | volume = 55 | issue = 2 | pages = 325–337 | last = Charlesworth | first = D. |author2=B. Charlesworth | title = Theoretical genetics of Batesian mimicry III. Evolution of dominance | journal = [[Journal of Theoretical Biology]] | year=1975 | pmid=1207162 | doi=10.1016/s0022-5193(75)80083-x| bibcode=1975JThBi..55..325C }}</ref> They argue that supergenes arose ''in situ''. This is known as Turner's sieve hypothesis.<ref>Turner, J. R. G. 1984.  "Mimicry: The Palatability Spectrum and its Consequences". In R. I. Vane-Wright, & P. R. Ackery (eds.), ''The Biology of Butterflies'', ch. 14.  "Symposia of the Royal Entomological Society of
London" ser., #11. London: Academic Pr.</ref> John Maynard Smith agreed with this view in his authoritative textbook,<ref name="Smith 1998" /> but the question is still not definitively settled.