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[[File:Autosomal dominant and recessive.svg|thumb|Autosomal dominant and autosomal recessive inheritance, the two most common [[Mendelian inheritance]] patterns. An [[autosome]] is any chromosome other than a [[sex chromosome]].|500px]]
<!-- Note that "dominant x" and "recessive x" (for many values of x) redirect to this article so "dominant" and "recessive" are currently regarded as an important part of the _subject_ of this article. Therefore please ensure these words appear bolded and as early as possible in the lead. Similarly for "autosomal dominant" and "autosomal recessive". -->
In [[genetics]], '''dominance''' is the phenomenon of one variant ([[allele]]) of a [[gene]] on a [[chromosome]] masking or overriding the [[Phenotype|effect]] of a different variant of the same gene on [[Homologous chromosome|the other copy of the chromosome]].<ref>{{cite web|title=dominance|url=http://www.oxforddictionaries.com/definition/english/dominance|archive-url=https://web.archive.org/web/20120718084053/http://oxforddictionaries.com/definition/english/dominance|url-status=dead|archive-date=July 18, 2012|work=Oxford Dictionaries Online|publisher=Oxford University Press|access-date=14 May 2014}}</ref><ref>{{cite web|title=express|url=http://www.oxforddictionaries.com/definition/english/express|archive-url=https://web.archive.org/web/20120718025722/http://oxforddictionaries.com/definition/english/express|url-status=dead|archive-date=July 18, 2012|work=Oxford Dictionaries Online|publisher=Oxford University Press|access-date=14 May 2014}}</ref> The first variant is termed '''dominant''' and the second is called '''recessive'''. This state of having [[Heterozygosity|two different variants]] of the same gene on each chromosome is originally caused by a [[mutation]] in one of the genes, either new (''de novo'') or [[Heredity|inherited]]. The terms '''autosomal dominant''' or '''autosomal recessive''' are used to describe gene variants on non-sex chromosomes ([[autosomes]]) and their associated traits, while those on [[sex chromosomes]] (allosomes) are termed [[X-linked dominant]], [[X-linked recessive]] or [[Y-linked]]; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child (see [[Sex linkage]]). Since there is only one copy of the [[Y chromosome]], Y-linked traits cannot be dominant or recessive.<ref>{{Cite journal |last1=Eggers |first1=Stefanie |last2=Sinclair |first2=Andrew |date=2012 |title=Mammalian sex determination—insights from humans and mice |journal=Chromosome Res |publication-place=Dordrecht |publisher=Springer-Verlag |volume=20 |issue=1 |pages=215–238 |doi=10.1007/s10577-012-9274-3 |pmid=22290220 |issn=0967-3849|hdl=11343/270255 |hdl-access=free }}</ref> Additionally, there are other forms of dominance, such as '''incomplete dominance''', in which a gene variant has a partial effect compared to when it is present on both chromosomes, and '''co-dominance''', in which different variants on each chromosome both show their associated traits.
Dominance is a key concept in [[Mendelian inheritance]] and [[classical genetics]]. Letters and [[Punnett squares]] are used to demonstrate the principles of dominance in teaching, and the upper-case letters are used to denote dominant alleles and lower-case letters are used for recessive alleles. An often quoted example of dominance is the inheritance of [[seed]] shape in [[pea]]s. Peas may be round, associated with allele '''''R''''', or wrinkled, associated with allele ''r''. In this case, three combinations of alleles (genotypes) are possible: '''''RR''''', '''''Rr''''', and '''''rr'''''. The '''''RR''''' ([[homozygous]]) individuals have round peas, and the '''''rr''''' (homozygous) individuals have wrinkled peas. In '''''Rr''''' ([[heterozygous]]) individuals, the '''''R''''' allele masks the presence of the ''r'' allele, so these individuals also have round peas. Thus, allele '''''R''''' is dominant over allele '''''r''''', and allele '''''r''''' is recessive to allele '''''R'''''.<ref>{{Cite book |last1=Bateson |first1=William |title=Mendel's Principles of Heredity: A Defence, with a Translation of Mendel's Original Papers on Hybridisation |last2=Mendel |first2=Gregor |date=2009 |publisher=Cambridge University Press |isbn=978-1108006132 |doi=10.1017/CBO9780511694462}}</ref>
Dominance is not inherent to an allele or its traits ([[phenotype]]). It is a strictly relative effect between two alleles of a given gene of any function; one allele can be dominant over a second allele of the same gene, recessive to a third, and [[#Co-dominance|co-dominant]] with a fourth. Additionally, one allele may be dominant for one trait but not others.<ref name=":0">{{Cite journal |last1=Billiard |first1=Sylvain |last2=Castric |first2=Vincent |last3=Llaurens |first3=Violaine |date=2021 |title=The integrative biology of genetic dominance |journal=Biol Rev Camb Philos Soc |location=Oxford, UK |publisher=Oxford, UK: Blackwell Publishing Ltd |volume=96 |issue=6 |pages=2925–2942|doi=10.1111/brv.12786 |pmid=34382317 |pmc=9292577 }}</ref> Dominance differs from [[epistasis]], the phenomenon of an allele of one gene masking the effect of alleles of a ''different'' gene.<ref>{{cite book |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK21249/ |title=Modern Genetic Analysis |chapter=Gene Interaction Leads to Modified Dihybrid Ratios |date=1999 |author=Griffiths AJF |author2=Gelbart WM |author3=Miller JH |isbn=978-0-7167-3118-4 |publisher=W. H. Freeman & Company |place=New York |display-authors=etal |url-access=registration |url=https://archive.org/details/moderngeneticana0000unse }}</ref>
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[[File:Journal of Agricultural Research (1917) (14582377398).jpg|thumb|240px|Inheritance of dwarfing in maize. Demonstrating the heights of plants from the two parent variations and their F1 heterozygous hybrid (centre)]]
Mendel did not use the terms gene, allele, phenotype, genotype, homozygote, and heterozygote, all of which were introduced later. He did introduce the notation of capital and lowercase letters for dominant and recessive alleles, respectively, still in use today.
In 1928, British population geneticist [[Ronald Fisher]] proposed that dominance acted based on natural selection through the contribution of [[modifier genes]].<!--[[natural selection]] was first proposed by the British population geneticist [[Ronald Fisher]] in 1928,<ref>Fisher, R.A. 1928. [http://digital.library.adelaide.edu.au/coll/special//fisher/68.pdf The possible modification of the response of the wild type to recurrent mutations] {{webarchive |url=https://web.archive.org/web/20090218135035/http://digital.library.adelaide.edu.au/coll/special/ |date=February 18, 2009 }}. Am. Nat., 62: 115–126.</ref> and expanded upon in his book ''[[The Genetical Theory of Natural Selection]]''.<ref>Fisher, R.A. 1930. ''The Genetical Theory of Natural Selection'', Clarendon Press, Oxford</ref> However, [[Sewall Wright]] and [[J.B.S. Haldane]] believed that the main explanation for dominance should be based on--> In 1929, American geneticist [[Sewall Wright]] responded by stating that dominance is simply a physiological consequence of metabolic pathways and the relative necessity of the gene involved.<ref>Mayo, O. and Bürger, R. 1997. [http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=637 The evolution of dominance: A theory whose time has passed?] {{Webarchive|url=https://web.archive.org/web/20160304073242/http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=637 |date=2016-03-04 }} "Biological Reviews", Volume 72, Issue 1, pp. 97–110</ref><ref>Bourguet, D. 1999. [http://www.nature.com/hdy/journal/v83/n1/full/6885600a.html The evolution of dominance] {{Webarchive|url=https://web.archive.org/web/20160829172824/http://www.nature.com/hdy/journal/v83/n1/full/6885600a.html |date=2016-08-29 }} ''Heredity'', Volume 83, Number 1, pp. 1–4</ref><ref>Bagheri, H.C. 2006. [https://web.natur.cuni.cz/zoologie/biodiversity/prednasky/EvolucniGenetika/clanky_2017/Bagheri%202006%20allele%20dominance%20review.pdf Unresolved boundaries of evolutionary theory and the question of how inheritance systems evolve: 75 years of debate on the evolution of dominance] {{Webarchive|url=https://web.archive.org/web/20190702000658/https://web.natur.cuni.cz/zoologie/biodiversity/prednasky/EvolucniGenetika/clanky_2017/Bagheri%202006%20allele%20dominance%20review.pdf |date=2019-07-02 }} "Journal of Experimental Zoology Part B: Molecular and Developmental Evolution", Volume 306B, Issue 4, pp. 329–359</ref><ref name=":0" />
==Types of dominance==
===Complete dominance (Mendelian)===
In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks are considered ''dominant'' to the other allele, and the masked allele is considered ''recessive''.<ref name=":1">{{Cite journal |last1=Rodríguez-Beltrán |first1=Jerónimo |last2=Sørum |first2=Vidar |last3=Toll-Riera |first3=Macarena |last4=de la Vega |first4=Carmen |last5=Peña-Miller |first5=Rafael |last6=San Millán |first6=Álvaro |date=2020 |title=Genetic dominance governs the evolution and spread of mobile genetic elements in bacteria |journal=Proc Natl Acad Sci U S A |location=United States |publisher=United States: National Academy of Sciences |volume=117 |issue=27 |pages=15755–15762 |doi=10.1073/pnas.2001240117 |doi-access=free |pmid=32571917 |pmc=7355013 |bibcode=2020PNAS..11715755R |issn=0027-8424}}</ref>
When we only look at one trait determined by one pair of genes, we call it '''[[Monohybrid cross|monohybrid inheritance]]'''. If the crossing is done between parents (P-generation, F0-generation) who are homozygote dominant and homozygote recessive, the offspring (F1-generation) will always have the heterozygote genotype and always present the phenotype associated with the dominant gene.
[[File:MonohydrideP.png|left|thumb|342x342px|Monohybrid cross between homozygote dominant (GG) and homozygote recessive (gg), always resulting in heterozygote genotype (Gg) and the phenotype associated with the dominant allele, in this case capital G. ]]
[[File:MonohydrideF1.png|thumb|340x340px|Monohybrid cross between heterozygotes (Gg), resulting in genptypical ratio 1:2:1 (GG:Gg:gg) and phenotypical ratio 3:1 (G:g).]]
However, if the F1-generation is further crossed with the F1-generation (heterozygote crossed with heterozygote) the offspring (F2-generation) will present the phenotype associated with the dominant gene ¾ times. Although heterozygote monohybrid crossing can result in two phenotype variants, it can result in three genotype variants - homozygote dominant, heterozygote and homozygote recessive, respectively.<ref>{{Cite journal |last1=Trudy |first1=F. C. Mackay |last2=Robert |first2=R. H. Anholt |date=2022 |title=Gregor Mendel's legacy in quantitative genetics |journal=PLOS Biology |publisher=Public Library of Science (PLoS) |volume=20 |issue=7 |pages=e3001692 |doi=10.1371/journal.pbio.3001692 |doi-access=free |pmid=35852997 |pmc=9295954 |issn=1544-9173}}</ref>
In '''[[Dihybrid cross|dihybrid]] inheritance''' we look at the inheritance of two pairs of genes simultaneous. Assuming here that the two pairs of genes are located at non-homologous chromosomes, such that they are not coupled genes (see [[genetic linkage]]) but instead inherited independently. Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively. The offspring (F1-generation) will always heterozygous and present the phenotype associated with the dominant allele variant.
[[File:DihydrideP.png|left|thumb|370x370px|Dihybrid cross between homozygote dominant (GGRR) and homozygote recessive (ggrr) always resulting in heterozygotes (GgRr) with phenotype associated with the dominant alleles G and R.]]
[[File:DihydrideF1.png|thumb|374x374px|Dihybrid cross between heterozygotes (GgRr), resulting in the phenotypical ratio 9:3:3:1 (G and R: G and r: g and R: g and r)]]
However, when crossing the F1-generation there are four possible phenotypic possibilities and the phenotypical [[ratio]] for the F2-generation will always be 9:3:3:1.<ref>{{Cite book |last1=Alberts |first1=Bruce |title=Essential cell biology |last2=Heald |first2=Rebecca |last3=Hopkin |first3=Karen |last4=Johnson |first4=Alexander |last5=Morgan |first5=David |last6=Roberts |first6=Keith |last7=Walter |first7=Peter |date=2023 |publisher=W.W. Norton & Company |isbn=9781324033394 |edition=Sixth edition.; International student}}</ref>
===
[[File:Incomplete dominance.svg|thumb|This [[Punnett square]] illustrates incomplete dominance. In this example, the red petal trait associated with the R [[allele]] recombines with the white petal trait of the r allele. The plant incompletely expresses the dominant trait (R) causing plants with the Rr genotype to express flowers with less red pigment resulting in pink flowers. The colors are not blended together, the dominant trait is just expressed less strongly.]]
{{See also|partial dominance hypothesis}}
Incomplete dominance (also called ''partial dominance'', ''semi-dominance'', ''intermediate inheritance'', or occasionally incorrectly ''co-dominance'' in reptile genetics<ref>{{cite web |url=https://reptilesmagazine.com/a-crash-course-in-reptile-genetics/ |title=A Crash Course in Reptile Genetics |last=Bulinski |first=Steven |date=2016-01-05 |website=[[Reptiles (magazine)|Reptiles]] |publisher=Living World Media |access-date=2023-02-03 |archive-url=https://web.archive.org/web/20200204020644/https://reptilesmagazine.com/a-crash-course-in-reptile-genetics/ |archive-date=2020-02-04 |quote=The term co-dominant is often used interchangeably with incomplete dominant, but the two terms
When plants of the F<sub>1</sub> generation are self-pollinated, the phenotypic and genotypic ratio of the F<sub>2</sub> generation will be 1:2:1 (Red:Pink:White).<ref name=":2">{{
===Co-dominance (non-Mendelian)===
[[File:Co-dominance Rhododendron.jpg|thumb|left|Co-dominance in a [[Camellia]] cultivar]]
[[File:ABO system codominance.svg|thumb|[[ABO blood group system|A and B blood types]] in humans show co-dominance, but the O type is recessive to A and B.]]
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Co-dominance occurs when the contributions of both alleles are visible in the phenotype and neither allele masks another.
For example, in the [[ABO blood group system]], chemical modifications to a [[glycoprotein]] (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (''I<sup>A</sup>'', ''I<sup>B</sup>'') and dominant over the recessive ''i'' at the [[ABO (gene)|ABO locus]]. The ''I<sup>A</sup>'' and ''I<sup>B</sup>'' alleles produce different modifications. The enzyme coded for by ''I<sup>A</sup>'' adds an N-acetylgalactosamine to a membrane-bound H antigen. The ''I<sup>B</sup>'' enzyme adds a galactose. The ''i'' allele produces no modification. Thus the ''I<sup>A</sup>'' and ''I<sup>B</sup>'' alleles are each dominant to ''i'' (''I<sup>A</sup>I<sup>A</sup>'' and ''I<sup>A</sup>i'' individuals both have type A blood, and ''I<sup>B</sup>I<sup>B</sup>'' and ''I<sup>B</sup>i'' individuals both have type B blood), but ''I<sup>A</sup>I<sup>B</sup>'' individuals have both modifications on their blood cells and thus have type AB blood, so the ''I<sup>A</sup>'' and ''I<sup>B</sup>'' alleles are said to be co-dominant.<ref name=":2" />
Another example occurs at the locus for the [[beta-globin]] component of [[hemoglobin]], where the three molecular phenotypes of ''Hb<sup>A</sup>/Hb<sup>A</sup>'', ''Hb<sup>A</sup>/Hb<sup>S</sup>'', and ''Hb<sup>S</sup>/Hb<sup>S</sup>'' are all distinguishable by [[protein electrophoresis]]. (The medical condition produced by the heterozygous genotype is called ''[[sickle-cell trait]]'' and is a milder condition distinguishable from ''[[sickle-cell anemia]]'', thus the alleles show ''incomplete dominance''
Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate – in reality, such cases should not be said to exhibit dominance at all.<ref name=":2" />
==Relationship to other genetic concepts==
Dominance can be influenced by various genetic interactions and it is essential to evaluate them when determining phenotypic outcomes. [[Allele|Multiple alleles]], [[epistasis]] and [[Pleiotropy|pleiotropic]] genes are some factors that might influence the phenotypic outcome.<ref name=":3">{{Cite journal |last=Ingelman-Sundberg |first=M. |date=2005 |title=Genetic polymorphisms of cytochrome P450 2D6 (CYP2D6): clinical consequences, evolutionary aspects and functional diversity |journal=Pharmacogenomics J |location=United States |publisher=United States: Nature Publishing Group |volume=5 |issue=1 |pages=6–13 |doi=10.1038/sj.tpj.6500285 |pmid=15492763 |s2cid=10695794 |issn=1470-269X}}</ref>
===Multiple alleles===
{{Main|Allele#Multiple alleles}}Although any individual of a diploid organism has at most two different alleles at a given locus, most genes exist in a large number of allelic versions in the population as a whole. This is called [[Polymorphism (biology)|polymorphism]], and is caused by mutations. Polymorphism can have an effect on the dominance relationship and phenotype, which is observed in the [[ABO blood group system]]. The gene responsible for human blood type have three alleles; A, B, and O, and their interactions result in different blood types based on the level of dominance the alleles expresses towards each other.<ref name=":3" /><ref>{{Cite journal |last1=Yamamoto |first1=F |last2=Clausen |first2=H |last3=White |first3=T |last4=Marken |first4=J |last5=Hakomori |first5=S |date=1990 |title=Molecular genetic basis of the histo-blood group ABO system |journal=Nature |volume=345 |issue=6272 |pages=229–233 |doi=10.1038/345229a0 |pmid=2333095|bibcode=1990Natur.345..229Y |s2cid=4237562 }}</ref>
=== Pleiotropic genes ===
{{Main|Pleiotropy}}
Pleiotropic [[gene]]s are genes where one single gene affects two or more characters (phenotype). This means that a gene can have a dominant effect on one trait, but a more recessive effect on another trait.<ref>{{Cite journal |last1=Du |first1=Qingzhang |last2=Tian |first2=Jiaxing |last3=Yang |first3=Xiaohui |last4=Pan |first4=Wei |last5=Xu |first5=Baohua |last6=Li |first6=Bailian |last7=Ingvarsson |first7=Pär K. |last8=Zhang |first8=Deqiang |date=2015 |title=Identification of additive, dominant, and epistatic variation conferred by key genes in cellulose biosynthesis pathway in Populus tomentosa |journal=DNA Res |location=England |publisher=England: Oxford University Press |volume=22 |issue=1 |pages=53–67 |doi=10.1093/dnares/dsu040 |pmid=25428896 |pmc=4379978 |issn=1340-2838}}</ref>
===Epistasis===
{{Main|Epistasis}}
Epistasis is interactions between multiple alleles at different loci. Easily said, several genes for one phenotype. The dominance relationship between alleles involved in epistatic interactions can influence the observed phenotypic ratios in offspring.<ref>{{Cite journal |last=Phillips |first=Patrick C |date=2008 |title=Epistasis - the essential role of gene interactions in the structure and evolution of genetic systems |journal=Nat Rev Genet |location=London |publisher=London: Nature Publishing Group |volume=9 |issue=11 |pages=855–867 |doi=10.1038/nrg2452 |pmid=18852697 |pmc=2689140 |issn=1471-0056}}</ref>
==See also==
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* [[Mitochondrial DNA]]
* [[Punnett square]]
* [[Summation theorems (biochemistry)]]
* [[Chimera (genetics)|Chimerism]]
==References==
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