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[[File:Polar over- VS. under-dominance.jpg|thumb|This figure depicts a generic graphical comparison of polar over dominance and polar under dominance. Differential inheritance is shown in a parent-of-origin type fashion in this case.]]
'''Polar overdominance''' is a unique form of inheritance originally described in livestock, with relevant examples in humans<ref name="Wermter 1126–1134">{{Cite journal|last1=Wermter|first1=Anne-Kathrin|last2=Scherag|first2=André|last3=Meyre|first3=David|last4=Reichwald|first4=Kathrin|last5=Durand|first5=Emmanuelle|last6=Nguyen|first6=Thuy Trang|last7=Koberwitz|first7=Kerstin|last8=Lichtner|first8=Peter|last9=Meitinger|first9=Thomas|date=2008-04-09|title=Preferential reciprocal transfer of paternal/maternal DLK1 alleles to obese children: first evidence of polar overdominance in humans|journal=European Journal of Human Genetics|volume=16|issue=9|pages=1126–1134|doi=10.1038/ejhg.2008.64|pmid=18398438|issn=1018-4813|doi-access=free}}</ref> and mice being discovered shortly after. The term polar is used to describe this type of [[overdominance]] because the [[phenotype]] of the [[Zygosity#Heterozygous|heterozygote]] is more prevalent than the other [[genotypes]]. This polarity is shown as differential phenotype is only present in one of the heterozygote configurations when the recessive [[allele]] is inherited in a parent of origin type fashion.<ref name=":1" /> Polar overdominance differs from regular overdominance (also known as [[heterozygote advantage]]) where both heterozygote genotypes display a phenotype that has increased fitness regardless of the parent of origin. Studying this type of inheritance could have practical applications in preventative medicine for humans as well as a variety of other agricultural applications.

== Discovery ==
The first described occurrence of polar overdominance in sheep was shown after finding that a [[mutation|mutant]] allele, called '''callipyge''' (after [[Venus Callipyge]]), must be inherited from the father to cause a condition called [[muscle]] [[Organ hypertrophy|hypertrophy]]. Muscle hypertrophy in the offspring is caused by an increase in the size and proportion of [[Skeletal_muscle#Skeletal_muscle_cells|muscle fibers]], namely the [[Skeletal_muscle#Twitch_speed|fast-twitch]] muscle fibers.<ref>{{Cite journal|last1=Georges|first1=M.|last2=Charlier|first2=C.|last3=Smit|first3=M.|last4=Davis|first4=E.|last5=Shay|first5=T.|last6=Tordoir|first6=X.|last7=Takeda|first7=H.|last8=Caiment|first8=F.|last9=Cockett|first9=N.|date=2004-01-01|title=Toward Molecular Understanding of Polar Overdominance at the Ovine Callipyge Locus|url=http://symposium.cshlp.org/content/69/477|journal=Cold Spring Harbor Symposia on Quantitative Biology|volume=69|pages=477–484|doi=10.1101/sqb.2004.69.477|issn=0091-7451|pmid=16117683|doi-access=free}}</ref> This increase is generally located in the hind quarters and torso. Muscle hypertrophy only manifests itself in the offspring approximately one month after birth.<ref name=":0">M Georges, N Cockett. The ovine callipyge locus: a paradigm illustrating the importance of non-Mendelian genetics in livestock. Reproduction, Nutrition, Development, EDP Sciences, 1996, 36 (6), pp.651-657.</ref> Polar overdominance shows evidence of an imprinted [[Locus (genetics)|locus]] displayed as the difference between the expression of heterozygote phenotypes in a parent-of-origin type fashion. It was discovered that a [[single-nucleotide polymorphism]] in the ''DLK1''–''DIO3'' imprinted [[gene cluster]] affects the gene expression of paternal allele-specific genes and several maternal allele-specific [[long non-coding RNA]] and [[microRNA]].<ref>{{Cite journal|last1=Bidwell|first1=C. A.|last2=Waddell|first2=J. N.|last3=Taxis|first3=T. M.|last4=Yu|first4=H.|last5=Tellam|first5=R. L.|last6=Neary|first6=M. K.|last7=Cockett|first7=N. E.|date=2014-08-01|title=New insights into polar overdominance in callipyge sheep|journal=Animal Genetics|volume=45|pages=51–61|doi=10.1111/age.12132|pmid=24990181|issn=1365-2052}}</ref> [[Ectopic expression]] of the [[DLK1|Delta-like 1 homologue (DLK1)]] and the [[RTL1|Retrotransposon-like 1 (RTL1/PEG11)]] genes which are paternally expressed proteins in skeletal muscle are a hallmark of these mutant individuals.<ref>{{Cite journal|last1=Xu|first1=Xuewen|last2=Ectors|first2=Fabien|last3=Davis|first3=Erica E.|last4=Pirottin|first4=Dimitri|last5=Cheng|first5=Huijun|last6=Farnir|first6=Frédéric|last7=Hadfield|first7=Tracy|last8=Cockett|first8=Noelle|last9=Charlier|first9=Carole|date=2015-10-16|title=Ectopic Expression of Retrotransposon-Derived PEG11/RTL1 Contributes to the Callipyge Muscular Hypertrophy|journal=PLOS ONE|volume=10|issue=10|article-number=e0140594|doi=10.1371/journal.pone.0140594|issn=1932-6203|pmc=4608697|pmid=26474044|bibcode=2015PLoSO..1040594X|doi-access=free}}</ref>

== Agricultural application ==
More and more studies have identified [[Quantitative trait locus|quantitative trait loci (QTL)]] that show evidence of [[genomic imprinting]] in farm animals other than sheep.<ref>{{Cite book|title=Encyclopedia of Genetics, Genomics, Proteomics and Bioinformatics|last=Groenen|first=Martien A. M.|date=2004-01-01|publisher=John Wiley & Sons, Ltd|isbn=978-0-470-01153-9|doi=10.1002/047001153x.g103311}}</ref> After polar overdominant inheritance was discovered to be the cause of muscular hypertrophy in sheep, the [[Sequence homology|ortholog]] for the human DLK1 gene (DLK1-GTL2 [[intergenic region]]) was studied in pigs to try to determine the effects of inheritance on ham weight. The original purpose of this study was to find the connection between genetics and ham weight to try to produce pigs that were abnormally large compared to the average. Before conducting this research, it was also hypothesized that the locus for ham weight was related to the [[ovine]] callipyge locus in sheep. After researching it was discovered that the two regions were likely unrelated due to different forms of parental inheritance exhibited in both cases and a relatively large physical distance between the loci on the chromosome. Unlike the form of paternal polar overdominance that occurs in the ovine callipyge locus, the locus that controls ham weight operates in a maternal polar overdominant fashion.<ref>{{Cite journal|last1=Boysen|first1=T. J.|last2=Tetens|first2=J.|last3=Thaller|first3=G.|date=2010-10-01|title=Detection of a quantitative trait locus for ham weight with polar overdominance near the ortholog of the callipyge locus in an experimental pig F2 population|journal=Journal of Animal Science|volume=88|issue=10|pages=3167–3172|doi=10.2527/jas.2009-2565|pmid=20581286|issn=1525-3163}}</ref>

== In humans ==
The term polar is used to describe this type of inheritance because the phenotype of one heterozygote is expressed at a level higher than other genotypes for the same locus including those displaying either homozygous geneotype.<ref name=":1">{{Cite journal|last1=Lawson|first1=Heather A.|last2=Cheverud|first2=James M.|last3=Wolf|first3=Jason B.|date=2013-09-01|title=Genomic imprinting and parent-of-origin effects on complex traits|journal=Nature Reviews Genetics|volume=14|issue=9|pages=609–617|doi=10.1038/nrg3543|pmc=3926806|pmid=23917626}}</ref> This unique form of inheritance has largely been studied in non-human mammals since 1996<ref name=":0" /> until it was first described in humans in 2008. In humans, the inheritance of the alleles for the [[DLK1]] gene (imprinted in [[Eutheria|eutherian mammals]]) is linked to a higher rate of obesity in the [[F1 generation]].<ref name="Wermter 1126–1134" /> The imprinted DLK1-GTL2 in sheep is homologous to the DLK1 gene in humans, and includes the callipyge locus.<ref>{{Cite journal|last1=Lewis|first1=Annabelle|last2=Redrup|first2=Lisa|title=Genetic Imprinting: Conflict at the Callipyge Locus|journal=Current Biology|year=2005|volume=15|issue=8|pages=R291–R294|doi=10.1016/j.cub.2005.04.003|pmid=15854893|doi-access=free}}</ref> There has been evidence to show that by screening potential fathers for a mutation at the DLK1 locus one could potentially see if their child is at a higher risk for obesity.<ref>{{Cite journal|last1=Huypens|first1=Peter|last2=Sass|first2=Steffen|last3=Wu|first3=Moya|last4=Dyckhoff|first4=Daniela|last5=Tschöp|first5=Matthias|last6=Theis|first6=Fabian|last7=Marschall|first7=Susan|last8=Angelis|first8=Martin Hrabě de|last9=Beckers|first9=Johannes|title=Epigenetic germline inheritance of diet-induced obesity and insulin resistance|journal=Nature Genetics|year=2016|volume=48|issue=5|pages=497–499|doi=10.1038/ng.3527|pmid=26974008|s2cid=205351366}}</ref> Individuals who inherit this mutant allele from their father are more likely to show signs of obesity because the DLK1 gene is key in [[adipogenesis]], or more simply the formation of fat cells.<ref>{{Cite journal|last1=Wang|first1=Yuhui|last2=Sul|first2=Hei Sook|date=2009-03-01|title=Pref-1 regulates mesenchymal cell commitment and differentiation through Sox9|journal=Cell Metabolism|volume=9|issue=3|pages=287–302|doi=10.1016/j.cmet.2009.01.013|issn=1932-7420|pmc=2673480|pmid=19254573}}</ref>

==See also==
*[[Imprinting (genetics)]]
*[[MEG3]] - a gene occasionally expressed through polar overdominance

== References ==
{{Reflist}}

==Further reading==
*{{cite journal |doi=10.1126/science.273.5272.236 |title=Polar Overdominance at the Ovine callipyge Locus |year=1996 |last1=Cockett |first1=Noelle E. |last2=Jackson |first2=Sam P. |last3=Shay |first3=Tracy L. |last4=Farnir |first4=Frédéric |last5=Berghmans |first5=Stéphane |last6=Snowder |first6=Gary D. |last7=Nielsen |first7=Dahlia M. |last8=Georges |first8=Michel |journal=Science |volume=273 |issue=5272 |pages=236–8 |pmid=8662506|bibcode=1996Sci...273..236C |s2cid=41491391 }}

[[Category:Genetics]]

Polar overdominance - История изменений

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