Editing Dominance (genetics) (section)

==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 |bibcode=2020PNAS..11715755R |doi=10.1073/pnas.2001240117 |issn=0027-8424 |pmc=7355013 |pmid=32571917 |doi-access=free}}</ref>

When we only look at one trait determined by one pair of genes, we call it '''[[Monohybrid cross|monohybrid inheritance]]'''.<ref name=":11">{{Cite web |date=2021-10-11 |title=18.4: Monohybrid Cross and the Punnett Square |url=https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Principles_of_Biology/02:_Chapter_2/18:_Patterns_of_Inheritance/18.04:_Monohybrid_Cross_and_the_Punnett_Square |access-date=2025-04-27 |website=Biology LibreTexts |language=en}}</ref> 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.<ref name=":12">{{Cite web |date=2019-10-01 |title=4.2.1: Monohybrid Crosses and Segregation |url=https://bio.libretexts.org/Courses/University_of_Arkansas_Little_Rock/Genetics_BIOL3300_(Leacock)/Genetics_Textbook/04:_Inheritance/4.02:__Mendelian_Genetics/4.2.01:_Monohybrid_Crosses_and_Segregation |access-date=2025-04-27 |website=Biology LibreTexts |language=en}}</ref>
[[File:MonohydrideF1.png|thumb|221x221px|Monohybrid cross between heterozygotes (Gg), resulting in genotypical 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.<ref name=":11" /> Although heterozygote monohybrid crossing can result in two phenotype variants, it can result in three genotype variants - &nbsp;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 |issn=1544-9173 |pmc=9295954 |pmid=35852997 |doi-access=free}}</ref><ref name=":12" />[[File:DihydrideF1.png|thumb|254x254px|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)|left]]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.<ref name=":13">{{Cite web |date=2016-06-02 |title=6.1: Dihybrid Crosses |url=https://bio.libretexts.org/Bookshelves/Genetics/Online_Open_Genetics_(Nickle_and_Barrette-Ng)/06:_Genetic_Analysis_of_Multiple_Genes/6.01:_Dihybrid_Crosses |access-date=2025-04-27 |website=Biology LibreTexts |language=en}}</ref> 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.<ref name=":13" /> 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><ref name=":13" />

===Incomplete dominance (non-Mendelian)===
[[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 |last=Bulinski |first=Steven |date=2016-01-05 |title=A Crash Course in Reptile Genetics |url=https://reptilesmagazine.com/a-crash-course-in-reptile-genetics/ |archive-url=https://web.archive.org/web/20200204020644/https://reptilesmagazine.com/a-crash-course-in-reptile-genetics/ |archive-date=2020-02-04 |access-date=2023-02-03 |website=[[Reptiles (magazine)|Reptiles]] |publisher=Living World Media |quote=The term co-dominant is often used interchangeably with incomplete dominant, but the two terms have different meanings.}}</ref>) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. The phenotypic result often appears as a blended form of characteristics in the heterozygous state. For example, the [[Antirrhinum majus|snapdragon]] flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the [[four o'clock flower|four o'clock plant]] wherein pink color is produced when true-bred parents of white and red flowers are crossed. In [[quantitative genetics]], where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit ''no dominance'' at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other.

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">{{Cite book |last=Brown |first=T. A. |title=Genomes 4 |date=2018 |publisher=Milton: Garland Science |isbn=9780815345084 |edition=4th |location=Milton |doi=10.1201/9781315226828 |s2cid=239528980}}</ref>

===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.]]
[[File:Co-dominance in Roan Cattle.svg|thumb|This Punnett square shows co-dominance. In this example a white bull (WW) mates with a red cow (RR), and their offspring exhibit co-dominance expressing both white and red hairs.]]

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'' concerning anemia, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are [[Transcription (genetics)|transcribed]] into [[RNA]].<ref name=":2" />

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" />