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'''Sex-linked barring''' is a plumage pattern on individual feathers in chickens, which is characterized by alternating pigmented and apigmented bars.<ref name=":0">{{Cite journal|last1=Hellström|first1=Anders R.|last2=Sundström|first2=Elisabeth|last3=Gunnarsson|first3=Ulrika|last4=Bed'Hom|first4=Bertrand|last5=Tixier-Boichard|first5=Michele|last6=Honaker|first6=Christa F.|last7=Sahlqvist|first7=Anna-Stina|last8=Jensen|first8=Per|last9=Kämpe|first9=Olle|date=2010-08-01|title=Sex-linked barring in chickens is controlled by the CDKN2A/B tumour suppressor locus|journal=Pigment Cell & Melanoma Research|language=en|volume=23|issue=4|pages=521–530|doi=10.1111/j.1755-148X.2010.00700.x|pmid=20374521|issn=1755-148X|url=https://hal.archives-ouvertes.fr/hal-01193403/file/2010_Hellstrom_Pigment_Cell_and_Melanoma_Research_1.pdf|doi-access=free}}</ref> The pigmented bar can either contain red pigment ([[Melanin|phaeomelanin]]) or black pigment ([[Melanin|eumelanin]]) whereas the apigmented bar is always white. The [[Locus (genetics)|locus]] is therefore often referred to as an 'eumelanin diluter' or 'melanin disruptor'.<ref>{{Cite book|title=An introduction to color forms of the domestic fowl: a look at color varieties and how they are made|last=Brian.|first=Reeder|date=2006-01-01|publisher=AuthorHouse|isbn=978-1-4259-0421-0|oclc=156823041}}</ref> Typical sex-linked barred breeds include the [[Plymouth Rock chicken|Barred Plymouth Rock]], Delaware, Old English Crele Games as well as [[Coucou de Rennes|Coucou de Renne]].<ref name=":1">{{Cite book|title=Poultry breeding and genetics|last=Crawford|first=R. D.|publisher=Elsevier|isbn=978-0-444-88557-9|oclc=956983183|year = 1990}}</ref>

== Plumage appearance ==

The presence of a white bar on a dark background is distinguishing sex-linked barring from Autosomal barring, another plumage pattern in chickens which is created by a black bar on a light color background (white/ beige or brown) as exemplified by the breed [[Faiyumi|Egyptian Fayoumi]].<ref name=":0" /> The absence of pigment in the white bar has been attributed to a lack of pigment producing cells ([[melanocyte]]s) in the feather follicle during feather growth.<ref>{{Cite journal|last=Bowers|first=RR|year=1984|title=Barred Plymouth Rock Melanocytes as a Possible Model for Vitiligo|journal=Yale Journal of Biology and Medicine|volume=3|page=340}}</ref><ref>{{Cite journal|last=Nickerson|first=Mark|date=1944-04-01|title=An experimental analysis of barred pattern formation in feathers|journal=Journal of Experimental Zoology|language=en|volume=95|issue=3|pages=361–397|doi=10.1002/jez.1400950305|issn=1097-010X}}</ref> Initially it was proposed that this lack was the result of cell death as a consequence of the ''B'' [[Locus (genetics)|locus]] mutation but later research demonstrated that the lack is the result of premature [[Cellular differentiation|cell differentiation]] rather than [[apoptosis]].<ref>{{Cite journal|last1=Lin|first1=S. J.|last2=Foley|first2=J.|last3=Jiang|first3=T. X.|last4=Yeh|first4=C. Y.|last5=Wu|first5=P.|last6=Foley|first6=A.|last7=Yen|first7=C. M.|last8=Huang|first8=Y. C.|last9=Cheng|first9=H. C.|date=2013-06-21|title=Topology of Feather Melanocyte Progenitor Niche Allows Complex Pigment Patterns to Emerge|journal=Science|language=en|volume=340|issue=6139|pages=1442–1445|doi=10.1126/science.1230374|issn=0036-8075|pmc=4144997|pmid=23618762|bibcode=2013Sci...340.1442L }}</ref>

Male chicken of traditional sex-linked barred breeds like the [[Plymouth Rock chicken|Barred Plymouth Rock]] usually show much wider and clearer white bands than females of the same breed.<ref name=":1" /> Further characteristics of sex-linked barred chickens are the dilution of skin pigment in the legs as well as a white dot at the top of the head of freshly hatched chicks which can be used for [[Chick sexing|autosexing]]: homozygous males have a much bigger spot than hemizygous females.<ref name=":1" />

== Genetics ==

Sex-linked barring has been established as the [[Dominance (genetics)|dominant]] locus ''B'' by traditional [[Mendelian inheritance|mendelian]] genetics in the beginning of the 20th century.<ref name=":1" /> The responsible gene was predicted to be located on the [[ZW sex-determination system|Z chromosome]]<ref>{{Cite journal|last=Bitgood|first=JJ|year=1988|title=Linear relationship of the loci for barring, der- mal melanin inhibitor, and recessive white skin on the chicken Z chromosome.|journal=Poult. Sci.|volume=67|issue=4|pages=530–533|doi=10.3382/ps.0670530|pmid=3165529|doi-access=free}}</ref><ref>{{Cite journal|last1=Dorshorst|first1=B. J.|last2=Ashwell|first2=C. M.|date=2009-09-01|title=Genetic mapping of the sex-linked barring gene in the chicken|journal=Poultry Science|language=en|volume=88|issue=9|pages=1811–1817|doi=10.3382/ps.2009-00134|pmid=19687264|issn=0032-5791|doi-access=free}}</ref> and since male birds are [[Heterogametic sex|homogametic]] (ZZ), they can be either [[Zygosity|hetero]]- or [[Zygosity|homozygous]] for sex-linked barring. Females are always [[Zygosity|hemizygous]] at this locus (ZW).
In 2010 Swedish scientist have identified four [[mutation]]s located in or around the [[Tumor suppressor gene|tumor suppressor]] [[Locus (genetics)|locus]] ''[[CDKN2A]]'', which appear to be associated with sex-linked barring.<ref name=":0" /> The four mutations are organized in 3 different [[allele]]s named ''B0'', ''B1'' and ''B2''. All alleles carry two [[Noncoding DNA|non-coding]] [[mutation]] located in regulatory regions of the gene (the [[Promoter (genetics)|promoter]] and [[intron]]) but only ''B1'' and ''B2'' carry two additional [[missense mutation]]s in a functional important domain of the [[protein]]. The ''B1'' [[allele]] is causing the typical sex-linked barring [[phenotype]]/ appearance and is present in most modern sex-linked barred chicken breeds. Females or male chickens carrying the ''B2'' [[allele]] in the [[Zygosity|heterozygous]] condition show a defined barring pattern but in the [[Zygosity|homozygous]] condition, males are essentially white with very little [[pigment]]ation.<ref name=":2">{{Cite journal|last=Van Albada|first=M|year=1960|title=Een geslachtsgebonden verdunningsfactor voor veerkleur bij Witte Leghorns.|journal=Nstituut voor de Pluimveeteelt}}</ref> This [[phenotype]] has been initially described as a distinct but closely related [[mutation]],<ref>{{Cite journal|last=Munro|first=SS|year=1946|title=A Sex-Linked True Breeding Blue Plumage Color.|journal=Poult. Sci.|volume=4|pages=408–9}}</ref> however, it was later assigned to the same [[gene]] and termed 'Sex-linked Dilution'.<ref name=":2" />

The ''B0'' allele only carries the two [[Noncoding DNA|non-coding]] [[mutation]]s and its contribution to the barring pattern remained unknown as it only occurred in breeds that also carry the Dominant white mutation which is masking the effect at the ''B'' [[Locus (genetics)|locus]].<ref name=":0" />). Recently scientists have removed the Dominant white [[mutation]] from chickens of those lines and were able to show that those chickens show a very light barring pattern. They named the phenotype 'Sex-linked Extreme Dilution'.<ref name=":3">{{Cite journal|last1=Thalmann|first1=Doreen Schwochow|last2=Ring|first2=Henrik|last3=Sundström|first3=Elisabeth|last4=Cao|first4=Xiaofang|last5=Larsson|first5=Mårten|last6=Kerje|first6=Susanne|last7=Höglund|first7=Andrey|last8=Fogelholm|first8=Jesper|last9=Wright|first9=Dominic|date=2017-04-07|title=The evolution of Sex-linked barring alleles in chickens involves both regulatory and coding changes in CDKN2A|journal=PLOS Genetics|volume=13|issue=4|article-number=e1006665|doi=10.1371/journal.pgen.1006665|pmid=28388616|pmc=5384658|issn=1553-7404 |doi-access=free }}</ref> As chickens with the ''B0'' allele show the weakest barring pattern compared to those that have the coding mutation in addition, the scientists propose an evolutionary scenario in which the [[Noncoding DNA|non-coding]] mutations occurred first and the two [[missense mutation]]s later in time and independently. As only the combination of both [[Noncoding DNA|non-coding]] and [[Missense mutation|missense]] mutations give the desired and pronounced barring pattern, ''B0'' alone is not present in modern Sex-linked barred breeds anymore.<ref name=":3" />

== Molecular pattern formation in the feather follicle ==
Scientists were able to show that either both or one of the [[Noncoding DNA|non-coding]] [[mutation]]s present in all ''B [[allele]]s'', cause an up-regulation of the activity of ''[[CDKN2A]]''.<ref name=":3" /> ). With more of the gene product, which is called [[CDKN2A|ARF]] (Alternate Reading frame Protein) in the cell, more of [[TP53|p53]] is protected from degradation. [[TP53|p53]] is a [[transcription factor]] which in turn activates more [[gene]]s involved in [[cell cycle]] regulation and [[apoptosis]]. As a consequence the cell stops dividing and starts to prematurely produce pigment.
The missense mutations in the ''B1'' and ''B2'' [[allele]], however, have an opposite effect.<ref name=":3" /> Both [[missense mutation]]s lead to a malfunctioning [[CDKN2A|ARF]] protein, which is counteracting the effect of the higher activity of the [[gene]] to some degree. The premature production of pigment is still obvious but less strong as observed in the ''B0'' allele. It is intriguing that the [[missense mutation]] in the ''B1'' [[allele]] is much more disruptive for the protein function than the one in the ''B2'' [[allele]] and the scientists believe that this is the reason for the observed [[Phenotype|phenotypic]] differences between those two [[allele]]s.

When the [[melanocyte]] [[Progenitor cell|progenitor]] cells start to migrate up from the bottom of the follicle into the [[Feather|barbs]] where they will make pigment, they further divide until a sufficient number of [[Melanocyte|pigment cells]] is achieved. As a consequence of the up-regulation of ''[[CDKN2A]]'', most cells will stop dividing and make new cells but instead start producing pigment- a black bar is emerging from the feather. But eventually, there will not be enough [[Melanocyte|pigment cells]] anymore. As they are recruited from the bottom of the feather follicle, the feather keeps on growing, creating the white bar. With the new set of pigment cells, the cyclic behavior starts again, creating alternating pigmented and apigmented bars.

== Sex-linked barring mutations and melanoma ==
Mutations in ''[[CDKN2A]]'' have been associated with the occurrence of familial [[melanoma]] in humans.<ref>{{Cite journal|last1=Dracopoli|first1=N. C.|last2=Fountain|first2=J. W.|date=1996-01-01|title=CDKN2 mutations in melanoma|journal=Cancer Surveys|volume=26|pages=115–132|issn=0261-2429|pmid=8783570}}</ref><ref>{{Cite journal|last1=Hussussian|first1=C. J.|last2=Struewing|first2=J. P.|last3=Goldstein|first3=A. M.|last4=Higgins|first4=P. A.|last5=Ally|first5=D. S.|last6=Sheahan|first6=M. D.|last7=Clark|first7=W. H.|last8=Tucker|first8=M. A.|last9=Dracopoli|first9=N. C.|date=1994-09-01|title=Germline p16 mutations in familial melanoma|journal=Nature Genetics|volume=8|issue=1|pages=15–21|doi=10.1038/ng0994-15|issn=1061-4036|pmid=7987387|s2cid=29732284}}</ref><ref>{{Cite journal|last1=Kannengiesser|first1=Caroline|last2=Dalle|first2=Stéphane|last3=Leccia|first3=Marie-Thérèse|last4=Avril|first4=Marie Françoise|last5=Bonadona|first5=Valerie|last6=Chompret|first6=Agnès|last7=Lasset|first7=Christine|last8=Leroux|first8=Dominique|last9=Thomas|first9=Luc|date=2007-08-01|title=New founder germline mutations of CDKN2A in melanoma-prone families and multiple primary melanoma development in a patient receiving levodopa treatment|journal=Genes, Chromosomes & Cancer|volume=46|issue=8|pages=751–760|doi=10.1002/gcc.20461|issn=1045-2257|pmid=17492760|s2cid=9324923 }}</ref> Changes in its gene product [[CDKN2A|ARF]] often cause the cell to lose their ability for [[Apoptosis|self-induced cell death]] or [[Cell cycle|cell cycle arrest]], which are mechanisms of cells to manage uncontrolled cell divisions and therefore the occurrence of cancer. It is intriguing that chickens carrying the ''B1'' or ''B2'' allele with a malfunctioning [[CDKN2A|ARF]] do not show any higher prevalence to any type of cancer and are usually considered very sturdy and easy to keep breeds. It is also astonishing that the majority of the egg and meat production industry is relying on chickens, which have a defect in a [[tumor suppressor gene]].<ref name=":3" />

== References ==
{{reflist}}

[[Category:Chicken plumage patterns]]
[[Category:Genetics]]

Sex-linked barring - История изменений

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