Editing Overdominance

{{More citations needed|date=December 2009}}

'''Overdominance''' is a phenomenon in genetics where the [[phenotype]] of the [[Zygosity|heterozygote]] lies outside the phenotypical range of both [[Zygosity|homozygous]] parents. Overdominance can also be described as [[heterozygote advantage]] regulated by a single [[Genomics|genomic]] [[Locus (genetics)|locus]], wherein [[heterozygous]] individuals have a higher [[Fitness (biology)|fitness]] than [[homozygous]] individuals. However, not all cases of the heterozygote advantage are considered overdominance, as they may be regulated by multiple genomic regions.<ref>{{Cite journal |last1=Charlesworth |first1=Deborah |last2=Willis |first2=John H. |date=November 2009 |title=The genetics of inbreeding depression |url=https://www.nature.com/articles/nrg2664 |journal=Nature Reviews Genetics |language=en |volume=10 |issue=11 |pages=783–796 |doi=10.1038/nrg2664 |pmid=19834483 |s2cid=771357 |issn=1471-0056|url-access=subscription }}</ref> Overdominance has been hypothesized as an underlying cause for [[heterosis]] (increased fitness of hybrid offspring).<ref>{{Cite journal |last1=Parsons |first1=P. A. |last2=Bodmer |first2=W. F. |date=April 1961 |title=The Evolution of Overdominance: Natural Selection and Heterozygote Advantage |url=https://www.nature.com/articles/190007a0 |journal=Nature |language=en |volume=190 |issue=4770 |pages=7–12 |doi=10.1038/190007a0 |pmid=13733020 |bibcode=1961Natur.190....7P |s2cid=4223238 |issn=0028-0836|url-access=subscription }}</ref><ref>{{Citation |last=Timberlake |first=W.E. |title=Heterosis |date=2013 |url=https://linkinghub.elsevier.com/retrieve/pii/B9780123749840007051 |encyclopedia=Brenner's Encyclopedia of Genetics |pages=451–453 |publisher=Elsevier |language=en |doi=10.1016/b978-0-12-374984-0.00705-1 |isbn=978-0-08-096156-9 |access-date=2022-11-04|url-access=subscription }}</ref>

[[File:Sickle cell disease overdominance and high fitness.jpg|thumb|Sickle cell anemia overdominance]]

== Examples ==

=== Sickle cell anemia ===
An example of overdominance in humans is that of the [[sickle cell anemia]]. This condition is determined by a single [[Polymorphism (biology)|polymorphism]]. Possessors of the deleterious allele have lower life expectancy, with [[Zygosity|homozygotes]] rarely reaching 50 years of age. However, this allele also yields some resistance to [[malaria]]. Thus in regions where malaria exerts or has exerted a strong selective pressure, sickle cell anemia has been selected for its conferred partial resistance to the disease. While homozygotes will have either no protection from malaria or a dramatic propensity to sickle cell anemia, [[Zygosity|heterozygotes]] have fewer physiological effects and a partial resistance to malaria.<ref>{{Cite journal |last1=Aidoo |first1=Michael |last2=Terlouw |first2=Dianne J |last3=Kolczak |first3=Margarette S |last4=McElroy |first4=Peter D |last5=ter Kuile |first5=Feiko O |last6=Kariuki |first6=Simon |last7=Nahlen |first7=Bernard L |last8=Lal |first8=Altaf A |last9=Udhayakumar |first9=Venkatachalam |date=April 2002 |title=Protective effects of the sickle cell gene against malaria morbidity and mortality |url=https://linkinghub.elsevier.com/retrieve/pii/S0140673602082739 |journal=The Lancet |language=en |volume=359 |issue=9314 |pages=1311–1312 |doi=10.1016/S0140-6736(02)08273-9|pmid=11965279 |s2cid=37952036 |url-access=subscription }}</ref>

=== Salmonoid major histocompatibility complex ===
[[Major histocompatibility complex]] (MHC) genes exhibit extensive variation, generally attributed to the notion of heterozygous individuals identifying a wider range of peptides than homozygous individuals. In [[arctic char]] population in [[Finland]], fish heterozygous for MHC alleles had fewer cysts, grew larger, and had a better chance at survival, all indicating a higher fitness of the heterozygotes.<ref name=":0">{{Cite journal |last1=Kekäläinen |first1=Jukka |last2=Vallunen |first2=J. Albert |last3=Primmer |first3=Craig R. |last4=Rättyä |first4=Jouni |last5=Taskinen |first5=Jouni |date=2009-09-07 |title=Signals of major histocompatibility complex overdominance in a wild salmonid population |journal=Proceedings of the Royal Society B: Biological Sciences |volume=276 |issue=1670 |pages=3133–3140 |doi=10.1098/rspb.2009.0727|pmid=19515657 |pmc=2817134 }}</ref>

=== ''Gymnadenia rhellicani'' colour polymorphism ===
In ''[[Gymnadenia rhellicani]]'', flower pigmentation is controlled by changes to [[amino acid]]s 612 and 663 in GrMYB1, which plays a role in [[anthocyanin]] pigment production. Red flowers, heterozygous with black and white alleles, maintain a reproductive fitness advantage over white and black varieties presumably because they attract both bee and fly pollinator populations. Since the emergence of the white allele, the frequency of the red phenotype has been increasing in wild populations in multiple regions of the alps.<ref name=":1">{{Cite journal |last1=Kellenberger |first1=Roman T. |last2=Byers |first2=Kelsey J. R. P. |last3=De Brito Francisco |first3=Rita M. |last4=Staedler |first4=Yannick M. |last5=LaFountain |first5=Amy M. |last6=Schönenberger |first6=Jürg |last7=Schiestl |first7=Florian P. |last8=Schlüter |first8=Philipp M. |date=2019-01-08 |title=Emergence of a floral colour polymorphism by pollinator-mediated overdominance |journal=Nature Communications |language=en |volume=10 |issue=1 |pages=63 |doi=10.1038/s41467-018-07936-x |pmid=30622247 |pmc=6325131 |bibcode=2019NatCo..10...63K |issn=2041-1723}}</ref>

== Polar overdominance ==
[[Polar overdominance]] is a type of overdominance where either only the paternal or maternal allele is being synthesized in the offspring. An example of this was illustrated by a famous [[Ram (sheep)|ram]] named Solid Gold and his offspring. This ram was known for its callipyge phenotype (pronounced muscular features and hindquarters) caused by a mutated allele, but only 15% of its offspring received these same traits. Solid Gold’s offspring only expressed the same callipyge phenotype if they inherited the mutated allele from Solid Gold and a wildtype allele from their mother, which would result in a C<sup>pat</sup>/N<sup>mat</sup> genotype. Offspring with genotypes such as: C<sup>pat</sup>/C<sup>mat</sup>, N<sup>pat</sup>/N<sup>mat</sup>, and N<sup>pat</sup>/C<sup>mat</sup> did not express the callipyge phenotype.<ref>{{Cite journal |last=Oczkowicz |first=M. |date=2009-01-23 |title=Polar overdominance – a putative molecular mechanism and the new examples in mammals |url=http://www.jafs.com.pl/Polar-overdominance-a-putative-molecular-nmechanism-and-the-new-examples-in-mammals,66362,0,2.html |journal=Journal of Animal and Feed Sciences |language=english |volume=18 |issue=1 |pages=17–27 |doi=10.22358/jafs/66362/2009 |issn=1230-1388|doi-access=free }}</ref>  

== Gillespie convention==
Population Geneticist [[John H. Gillespie]] established the following convention:<ref>Gillespie 2004</ref>
{| class="wikitable"
|-
| Genotype:
| A<sub>1</sub>A<sub>1</sub>
| A<sub>1</sub>A<sub>2</sub>
| A<sub>2</sub>A<sub>2</sub>
|-
| Relative fitness:
| 1
| 1-hs
| 1-s
|}
Where h is the heterozygote effect and s is the recessive allele effect.
Thus given a value for s (i.e.: 0<s<1), h can yield the following information:
{| class="wikitable"
|-
| h=0
| A<sub>1</sub> dominant, A<sub>2</sub> recessive
|-
| h=1
| A<sub>2</sub> dominant, A<sub>1</sub> recessive
|-
| 0<h<1
| incomplete dominance
|-
| h<0
| overdominance
|-
| h>1
| [[Underdominance]]
|}

For the case of sickle cell anemia the situation corresponds to the case h<0 in the Gillespie Model.

==See also==
*[[Dominance versus overdominance]]
*[[Polar overdominance]]
*[[Underdominance]]
*[[Outbreeding depression]]

==References ==
{{Reflist|1}}

==References==
* {{cite book
  |last = Gillespie
  |first = John
  |authorlink = John_H._Gillespie
  |title = Population Genetics: A Concise Guide, Second Edition
  |publisher = [[Johns Hopkins University Press]]
  |year = 2004
  |isbn = 0-8018-8008-4}}

[[Category:Classical genetics]]
[[Category:Genetics concepts]]