Colour Inheritance in Sheep - Rare Breeds Survival Trust

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Colour Inheritance in Sheep - Rare Breeds Survival Trust

Ark|FeatureColourinheritancein sheepHave you ever wonderedwhy a black calf suddenlyappears in a herd ofpristine White Parkcattle, red or mooritlambs in a flock of uniformly blackHebrideans, or black flecks in thered coat of some Tamworth pigs?Such annoying aberrations, whichmay be most inconvenient andupset your breeding plans, are notthe outcome of a witch's spell, andmaybe not even the result of illicitintrogression by a marauding male,but more probably the logicaloutcome of the genetic make-up ofyour animals.The coat colour of the wildancestors of our farm livestock wasdetermined primarily by theimportance of camouflage forsurvival, and necessarily in mostcases was dull and unspectacular.Domestication largely removed thethreat of predation, and thuspermitted the expression of a widerrange of colours. In some cases it islikely that unusual colours likepalomino, blue heads and whitebelts may have been preferred byowners of livestock, and hence wesee a wide variety of colours inmodern breeds, especially in poultry,horses and sheep.Pre-Mendel farmers (includingBakewell and the Colling brothers)bred their livestock without thebenefit of a knowledge of geneticsbut even then could fix distinctivecolour patterns such as the WhiteMulticolouredNorth Ronaldsaysheep on Lihou(CI) 1977.Park's black points or the Hereford'swhite face. Now that the inheritanceof colour is more clearly understood,breeding programmes canaccommodate the desire to producespecific colours, and can explainblack White Parks, whiteHebrideans, spotted Tamworths,and even the case of two whitesheep producing a lamb with Soaycolour and markings.ResearchThe earliest significant work wasprobably done with poultry, andthere is a long section on 'plumagecolour' in 'poultry breeding', firstpublished by MAFF in 1950. A morerecent review is given by Scrivener(2005), who refers to the earlier28Winter 2006 The Ark


Feature|ArkBy Lawrence Alderson.work of Carefoot (1985). Aconsiderable amount of researchhas also been done on equines.There is a detailed chapter on 'CoatColour and Texture' in 'EquineGenetics & Selection Procedures'(Wagoner, 1978), and Ann Bowling(who contributed a paper at the 1stRBI Congress) of VeterinaryGenetics Laboratories in USA hasalso done much work. Lessprogress has been made with cattlebut there is a useful website(Schmutz, 2005) with information onseveral breeds. There has been asurge of interest in pig colourgenetics recently, particularly as partof the EU project PigBioDiv1(Alderson and Plastow 2004) to suchan extent that some breed-definingcoat colour alleles have beenpatented.However, apart from poultry,sheep have probably received thegreatest attention. The greatvariety of colours and patternsfound in the species stimulatedinterest, and the major authorityis Stefan Adalsteinsson whoproduced an influential researchpaper on Icelandic sheep in 1970.He summarised the work in akeynote address at York in 1994,and several other authors (suchas Ryder, 1994) have written onthe subject. COGNOSAG(established at the 1st ColouredSheep Congress in Adelaide in1979 and currently administeredby JJ Lauvergne in France) is thebody responsible for identifyingand classifying the loci/allelescontrolling colour.Cross-speciesThere are considerable betweenspeciessimilarities in theinheritance of colour, and thus itis possible to cross-referenceresearch work in differentspecies. For example, in severalspecies the E (Extension) locus isresponsible for extendingpigment in the coat, while allelesat the A (Agouti) locus determinecolour patterns (ie, distribution ofpigment over the body). At thesame time, there are importantdifferences, not only betweenspecies, but also between breedswithin a species. For example,dun colour in cattle is caused bythree different alleles in Highland,Dexter and Galloway.This article will only skim thesurface of the subject – a fulldescription would take a volume– and will focus on sheep, but Iwould refer readers to thevarious authorities mentionedherein, and future articles cancover other aspects if readerswould find that useful.Basic geneticsColour in sheep is controlledprimarily at three loci (individualsites on a chromosome) – B, Eand A. There are others, such as S(Spotting) and C (Albino), but theyare of less importance except forone or two breeds. The B locuscontrols the production ofeumelanin – a pigment whichcauses black or chocolate-browncolour. The E locus determineswhether eumelanin can beexpressed, or whether it will bepartly replaced by phaeomelaningiving diluted colours (red or tan).The A locus determines the patternof colours, and possesses manyoptions through a variety of alleles(the coded units at each locuswhich determine development). Theinteraction between these loci canbe complex, illustrated by breedssuch as the Shetland which has avariety of colours combined in 30different patterns, but the principlesare relatively simple.Two alleles are found at eachlocus, one inherited from the sireand one from the dam. If they arethe same, it is homozygous; if theyare different it is heterozygous.There is a hierarchy of alleleswhereby dominant alleles inhibit theexpression of recessive alleles. Thusan animal may carry two differentalleles at a particular locus (ie, it isheterozygous), but the effect of onlyone of them will be expressed.These are shown as the locusfollowed by the allele (eg, A g – locusA and allele g for grey). If the mainloci mentioned above are shown thegenotype might be A a A a B B B b E + E +which would be a black sheepbecause of the presence of B B .Often these are set out in a matrixas shown in The Chance to Survive(Alderson, 1994a) although thesymbols used in that example havenow been superseded.Currently, among the 21 differentalleles found at the A locus, A Wh(white) is the most dominant and A a(non-agouti) the most recessivewith the others ranked betweenthem. In contrast, the B and E locieach have only two relevant allelesfor sheep. At B there is B B which isblack, and B b which is chocolatebrown.At E there is E D (dominantblack) and E + (recessive).The Ark Winter 2006 29


Ark|FeatureColoured breeds of sheepNative British breeds vary greatly inthe variety of colour they possess.Some breeds, such as ManxLoaghtan, Hebridean and BlackWelsh Mountain are a single solidcolour (they are known as wholecolouredor self-coloured). Othersare single-pattern breeds (ie, eachshows only one colour pattern) andinclude the Soay, Castlemilk Moorit,Jacob, Torddu/Torwen and Balwen.Only the Shetland and NorthRonaldsay possess a profusion ofcolour and patterns, although theBoreray patterns are also irregularand varied. Let us look briefly atsome of these breeds to illustratethe inheritance of colour in sheep.Other breeds might beconsidered, but are probably bestleft for the time being. Some breedslose the colour of birth with maturity.The Herdwick is born black, andsome Portlands are born red, butboth are white-wooled as adults.Other white breeds carry colouredrecessives; usually a black recessivein longwool breeds, and a Torddulikepattern in Down breedsWhole-coloured breedsMoorit is recessive to other colours.Therefore moorit animals cannotcarry other colours, otherwise theywould mask the moorit. Thus theManx Loaghtan genotype isA a A a B b B b E + E + (B b is chocolate-brownor moorit) whereas the Hebridean,varying from it only at the B locus,may be A a A a B B B b E + E + (BB is black).However, black can be achieved by adifferent route. E D is carried by theBlack Welsh Mountain and producesdominant black which over-rides theeffects of alleles at the A and B loci.Dominant black has found its wayinto the Hebridean, probably fromthe Black Welsh Mountain andJacob and, as the E D allele does notallow the B B allele to function, itprevents the ability to distinguishvisually the source of black in anyanimal. E D could be eliminated onlyby extensive test-mating, but that isa much wider subject. Nor does thissimplistic description of wholecolouredbreeds take into accountSoay ewe withextensive whitemarkings.Photo by SFurness.complications such as epistasis,incomplete dominance, andvariations such as white marks,sometimes seen in Manx Loaghtanand Hebridean sheep, which arecaused by spotting alleles at the Slocus.The Manx Loaghtan genotypetheoretically is the moststraightforward for breeders tomaintain. The genotype for moorit isrecessive, and therefore shouldbreed true. In contrast, blackHebridean sheep could carry mooritas a recessive and it might beexpressed if an animal inherited B bfrom both parents. In practicecomplications occur. There mayhave been introgression, legitimateor illicit. When Caesar Bacon wasestablishing his foundation flock ofManx Loaghtan a century ago, heintroduced some Shetland breedinganimals (Wade-Martins, 1990) and,although the influence was slight,some alleles may have persisted.Single pattern breedsMost Soay sheep carry a pattern oflight-coloured rump patch, belly, andfacial and leg markings (often knownas moufflon) on a chocolate-brownbackground (67.5% on Hirta). Thegenotype is A W A W B b B b E + E + , whereA W produces the pattern, B b ischocolate-brown, and E + permits theexpression of A and B factors.Other variations include themoufflon pattern on a tanbackground where a higher level ofphaeomelanin has been produced(22.5%), white markings (c.5%) asshown in the pictured ewe, andwhole-coloured (c.5%) – eitherchocolate-brown (see ManxLoaghtan genotype) or tan. Thelatter, which is very rare, isassociated with lower growth rateand higher mortality.Examples of association of colourwith other characteristics occur inmany species. The negative aspectseen in whole-coloured tan Soays, isalso seen in the white heifer diseasein Shorthorn cattle, and in lethaleffect of homozygous yellow inmice and homozygous roan inhorses. Adalsteinsson's research inthe 1950s and 1960s shows a morepositive aspect whereby colouredIcelandic sheep had better fertility/prolificacy than white sheep(Adalsteinsson, 1970).The Castlemilk Moorit, Torddu('black belly', also known asBadgerface) and Torwen ('whitebelly') have a similar pattern to theSoay, and it may be carried as arecessive by white sheep. A Soaymarkedlamb resulted from mating a30Winter 2006 The Ark


Feature|Arkwhite Welsh Halfbred ewe to a white Down ram(Alderson, 1994b). Both carried the B b allele(chocolate-brown), and the A W allele alongside theAWh allele. By chance the lamb received a doubledose of both B b and A W making it homozygous forSoay colour and pattern. The colour of the Balwenand Jacob is an expression of the spotting gene withwhite marks of varying extent on black, limited to thepoints in the Balwen but extensive in the Jacob. TheJacob is interesting because it carries dominant black(E D ) which is inhibited through epistasis by thespotting gene, but which appears in crossbred Jacobprogeny except when the other parent is a pink-nosedbreed such as the Dorset Horn.Multicoloured breedsTwo breeds show an arresting display of colour andpatterns. A survey of North Ronaldsay sheep on LingaHolm by Pilkington in 1979 showed that white andgrey were the most common colours. This would beexpected because of their dominance over othercolours, but the latter (black, moorit, tan, roan, etc.)were present in small numbers together with variouspatterns (Alderson, 1994c). Based on the number ofcoloured animals, it can be calculated that perhaps asmany as 40% of the flock might have been expectedto carry the recessive alleles for minor colours.The Shetland has an even greater array of coloursand patterns, which have been codified but may stillperplex the layman! Is the ram pictured a Smirslet? Awebsite (Baker, 2005) identifies the colours of Shetlandsheep and 30 different patterns, many known bywords derived from Old Norse. Some correspond topatterns seen in other breeds. For example, Katmoget(A b ) and Gulmoget (A t ) have the same genotype asTorddu and Torwen respectively (and A t is very similarto the Soay A W ), but the great majority are evidence ofthe presence of spotting alleles. Krunet, Bleset andSokket have similarities to the Balwen pattern.However, the overall hierarchy of dominance at the Alocus is White (A Wh ), Grey (A g ), Katmoget (A b ) andGulmoget (A t ), and finally Non-agouti (A a ), as for otherbreeds.No matter whether you are breeding sheep such asManx Loaghtan with a solid recessive colour, orShetlands with unusual colours in multiple patterns,the rules governing their selection are the same. Wehave come a long way since Gregor Mendelexperimented with sweet peas, but we can now usethe principles of genetics evolved by him to ouradvantage in breeding sheep. The variety of colour thatwe see in both field and showring is part of theirattraction and each breeder now has the ability tounderstand best how to achieve their chosenobjective. The reward may be realised in the higherproductivity conferred by coloured genes, or in achampionship sash on the back of a sheep whosecolour caught the judge’s eye.Shetland ram atWalls Show2000. Photo:CountrywideLivestock Ltd.References Adalsteinsson S. 1970. Colour inheritance in Icelandic Sheep and Relation betweenColour, Fertility and Fertilisation. J. Ag. Res. Iceland v.2, pp 3-135. Adalsteinsson S. 1994. Keynote address. In: L. Alderson (Ed), Proc. World Congress ofColoured Sheep, pp 4-15 Alderson, GLH. and Plastow, G.S. 2004. Use of DNA markers to assist with producttraceability and pedigree and their role in breed conservation. AGRI 35, pp 1-7 Alderson L. 1994a. The Chance to Survive (3rd edition); p 157 Alderson L. 1994b. The Chance to Survive (3rd edition); p 71 Alderson L. 1994c. The Chance to Survive (3rd edition); p 81 Baker K. 2005. www.shetlandsheepinfo.com/FLEECE/markings Carefoot, C. 1985. Creative Poultry Breeding. Ryder ML. 1994. Colour inheritance in rare sheep. The Ark, Vol XXI, No 6, pp 214-215. Schmutz S. 2005. Genetics of Coat Colour in Cattle. sask.usask.ca/~schmutz/colors.html Scrivener D. 2005. Exhibition Poultry Keeping. Wade-Martins P. 1990. The Manx Loghtan Story. The Decline and Revival of a PrimitiveBreed; p 11 Wagoner D. (Ed). 1978. Equine Genetics and Selection Procedures.Ascott The Smallholders StoreComplete Home HatcheryEverything you need—just add your own eggsFrom £99.99ATV Tipping Trailer500 litre capacity. £605.12Head GateHolds Sheep, Goatsand Calves fast.£49.99Glass Bottlesand JarsFREE MAIL ORDER CATALOGUE - JUST ASKAll prices include VAT but not deliveryAscott Smallholding Supplies LtdThe Old Creameryeamery, Four Crosses,LLANYMYNECH SY22 6LPCall Us for fLess - Local Rate Call 0845 130 6285Fax 0870 774 0140Browse and Securely ely Shop Online www.ascott.bizThe Ark Winter 2006 31

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