A l a b a m a A & M a n d A u b u r n U n i v r s i t i s Basic Hors Gntics ANR-1420 Undrstanding th basic principls of gntics and gn-slction mthods is ssntial for popl in th hors-brding businss and is also bnficial to any hors ownr whn it coms to making dcisions about a hors purchas, suitability, and utilization. Bfor gtting into th basics of hors-brding dcisions, howvr, it is important that brdrs undrstand th following trms. Chromosom - a rod-lik body found in th cll nuclus that contains th gns. Chromosoms occur in pairs in all clls, th xcption of th sx clls (sprm and gg). Horss hav 32 pairs of chromosoms, and donkys hav 31 pairs. Gn - a small sgmnt of chromosom (DNA) that contains th gntic cod. Gns occur in pairs, on on ach chromosom of a pair. Allls - th altrnativ stats of a particular gn. Th gn locatd at a fixd position on a chromosom will contain a particular gn or on of its allls. Multipl allls ar possibl. Gnotyp - th gntic makup of an individual. With allls A and a, thr possibl gnotyps ar AA, Aa, and aa. Not all of ths pairs of allls will rsult in th sam phnotyp bcaus pairs may hav diffrnt mods of action. Phnotyp - charactristics of an animal that can b sn or masurd for xampl, color, birth wight, spd. Qualitativ traits - traits that ar controlld by a singl pair or a fw pairs of gns. Qualitativ traits ar asily sortd into distinct catgoris and ar not highly affctd by nvironmntal influncs; thrfor, it is usually simpl to dtrmin an animal s gnotyp for a particular qualitativ trait. An xampl of a qualitativ trait in horss is chstnut vrsus black coat color. Quantitativ traits - traits that show a continuous rang of phnotypic variation. Quantitativ traits usually ar controlld by mor than on gn pair and ar havily influncd by nvironmntal factors, such as track condition, trainr xprtis, and nutrition. Bcaus of ths conditions, quantitativ traits cannot b classifid into distinct catgoris. Oftn, th important conomic traits of livstock ar quantitativ for xampl, cannon circumfrnc and racing spd. Hritability - th portion of th total phnotypic diffrnc (variation) among animals that is causd by th part of its gntic makup that can b passd from gnration to gnration. Homozygous - an individual whos gns for a particular trait ar th sam. For xampl, a black hors may b homozygous dominant for th black gn (), whil a chstnut hors is homozygous rcssiv for that gn (). Htrozygous - an individual whos gns for a particular trait ar diffrnt. For xampl, a phnotypically black hors may b htrozygous () for th black gn. www.acs.du
Slcting Qualitativ Traits: Basic Color Gntics Slcting qualitativ traits is a good plac to rviw som basic gntic principls bcaus most hors colors ar controlld by rlativly fw gns and ar asily idntifid phnotypically. Th ky to basic gntics is to rmmbr that th hors contributs only on gn for any alll that h or sh has to th offspring. For xampl, th bas coat color of any hors may b ithr black a gnotyp of or or chstnut (sorrl) a gnotyp of. Th homozygous black hors () can contribut only an gn to th offspring. Th homozygous chstnut hors () can contribut only th gn to th offspring. Howvr, th htrozygous black hors () can contribut ithr an or to th offspring. This is an xampl of simpl dominant gn action. An asy way to dtrmin th chancs of gtting a black or chstnut hors is to simply writ down th possibl gns contributd from both parnts (in bold lttrs blow) in a Punntt squar as shown: Dominant gn - an alll that is xprssd whn carrid by only on of a pair of chromosoms. For xampl, th alll for th black vrsus chstnut coat color is dominant, whil is rcssiv. Horss that hav on copy of th dominant alll ( or ) will b black unlss that color is modifid by othr gns. Rcssiv gn - an alll that is xprssd only whn th dominant alll is absnt for xampl, th alll for th black vrsus chstnut coat color. Horss that hav th alll on both chromosoms of a pair () will b chstnut unlss that color is modifid by othr gns. Additiv a gn ffct that occurs whn th htrozygous phnotyp is intrmdiat btwn th two homozygous phnotyps for xampl, aa = no product; Aa = product; AA= two tims th product. Many conomically important traits in livstock ar influncd by many pairs of additiv gns. Locus th plac on a chromosom whr a gn is locatd. Th plural of locus is loci. pistasis on locus masks, or controls, th xprssion of anothr locus. A locus, or loci, controlling th arly stps in a pathway can b pistatic to gns occurring latr in th pathway. For xampl, in horss, if an arly stp in a pathway that producs pigmnt (color) prcursors is maskd by th dominant whit gn, it dos not mattr what bas color th animal was supposd to b it will hav a whit coat. 2 Alabama Cooprativ xtnsion Systm black () black () = all offspring homozygous black () black () black () = ¼ homozygous black (), ½ htrozygous black () ¼ chstnut () black () chstnut () = all offspring htrzygous black () black () chstnut () = ½ htrozygous black () and ½ chstnut () black () black () = all offspring black; ½ homozygous (), ½ htrzygous () chstnut () chstnut () = all offspring chstnut ()
Th problm lis idntifying whthr th parnt th black bas coat is homozygous or htrozygous for that trait. Sinc w know that chstnut horss can only b homozygous rcssiv (), prforming tst matings of a black hors to chstnut horss will giv th brdr a clu to th black hors s gnotyp. Th mor matings that ar prformd in which no chstnut offspring ar producd, th mor assuranc thr is that th black hors is homozygous (). If vn on chstnut hors is producd, w know that th black hors has to b htrozygous (). Rcnt mapping of th hors s gnom has rducd this tdious procss to a simpl DNA diagnostic tst prformd on sampls of th hors s hair roots. Most brds includ both black and chstnut horss; howvr, som brds hav bn slctd for only th dominant alll, such as Frisians and Clvland Bays, and othrs hav bn slctd for only th rcssiv, such as Suffolks and Haflingrs. Th nxt obvious qustion is, if th bas coat color of a hors is ithr black or chstnut, why ar thr so fw tru black horss? It is bcaus thr ar othr diluting gns that rstrict or dilut th bas coat color. For xampl, th bay (agouti) gn (A), which has svral allls (A, a+ and a), rstricts th black color to th points (th lgs, ar rims, man, and tail). Dpnding on th dos of th bay gn rcivd by th hors from its parnts, a black hors can rmain a tru black (aa or aa) or may bcom bay (strong dilution at last on parnt contributing an A) or sal brown (lss dilution than bay at last on parnt contributing an a+). It is not clar how, or vn if, this bay dilution gn intracts th chstnut bas coat. Anothr xampl of a diluting gn is th dun (D) gn, which commonly is found in stock-typ horss, ponis, and th Norwgian Fjord. Th homozygous dominant (DD) or htrozygous (Dd) combination of this gn diluts th bas coat to grullo if th hors is black, to dun if th hors is bay, and to rd dun if th hors is chstnut. Th D gn also contributs a darkr dorsal strip and oftn darkr shouldr and lg bars. Th cram gn (c cr ) also diluts th coat color and has an additiv, or dosag, ffct. A singl dos (Cc cr ) a chstnut coat color rsults in a palomino and a bay color rsults in a buckskin. A singl dos of th cram gn on a black or sal brown coat rsults in a hors that is phnotypically similar to black or sal brown but is trmd a smoky black by som organizations. A c cr contributd from ach parnt (a doubl dos, c cr c cr ) diluts th chstnut color to crmllo (ivory coat, pink skin, blu ys) and th bay color to prlino (ivory coat, pink skin, blu ys, and a darkr man and tail). Th silvr dappl gn (Z) is also a diluting gn that rsults in a dark chocolat color a black coat, a silvr-mand bay on a bay coat, and no ffct on a chstnut coat. Horss th silvr dappl gn do not hav to b dappld, and th gn is most commonly found in Rocky Mountain horss and Shtland and Iclandic ponis and is rarly sn in Quartr Horss, Morgans, and Pruvian Pasos. Othr gns that dilut th bas coat color ar th Champagn (Ch) gn and th Parl (Prl) gn. Champagn is a dominant gn that can dilut any coat color or modification of th bas colors. Champagn diluts a black bas coat to brown and a chstnut coat to gold. Champagn horss hav ambr ys and lavndr-colord skin that bcoms spckld darkr pigmntd spots as th hors ags. Th Champagn dilution is common in Tnnss Walking Horss, Missouri Fox Trottrs, Miniaturs, and Spanish Mustangs. Th Parl gn is a rar dilution that is rcssiv. That is, it taks two doss of th gn to chang a bas chstnut coat to a light apricot color. A singl dos of th gn dos not affct th hors s bas coat color unlss it is combind th cram gn; thn, th rsulting coat color is phnotypically similar to th doubl cram dilution (prlino, crmllo), but in ffct, th hors is a psudo-crmllo or psudo-prlino. That is, th color looks lik crmllo or prlino, but it is causd by a diffrnt gntic action. Th Parl gn is found in Andalusians, Lusitanos, Quartr Horss, and Paints. In Quartr Horss and Paints, it historically has bn rfrrd to as th Barlink Factor. Anothr form of coat color modification is th gry gn (G). Th homozygous dominant (GG) and htrozygous dominant (Gg) both rsult in a hors that progrssivly dvlops mor whit hairs in its coat ag. Gry horss ar born a normal, or almost normal, coat color and vntually turn whit advancing ag. Th skin and ys of gry horss rmain dark. Bcaus th gry gn has a dominant gn action, at last on parnt of a gry hors must b gry. Gry occurs in many brds of horss, and it is th prdominant color in svral brds such as Lippizans and Andalusians. Mlanomas (skin tumors) ar mor common in gray horss. Whil th condition may b disfiguring, most of th tumors ar bnign. Basic Hors Gntics 3
Basic colors and rsults from common modifications or dilutions Bas Color Modifid/dilutd Rsult Furthr modifid/ dilutd Rsult black aa black dd black black AA, Aa +, Aa bay dd bay black a + a +, a + a sal brown dd sal brown black AA, Aa +, Aa bay DD or Dd dun black a + a +, a + a sal brown DD or Dd dun or brown grullo black aa black DD or Dd grullo chstnut dd chstnut chstnut DD or Dd rd dun black aa black CC black black aa black Cc cr smoky black black AA, Aa +, Aa bay Cc cr buckskin black AA, Aa +, Aa bay c cr c cr prlino black a + a +, a + a sal brown Cc cr dilutd sal brown smoky black chstnut CC chstnut chstnut Cc cr palomino chstnut c cr c cr crmllo black aa black zz black black aa black ZZ or Zz chocolat silvr man and tail black AA, Aa +, Aa bay ZZ or Zz bay lightnd points and silvr man and tail chstnut zz chstnut chstnut ZZ or Zz chstnut Champagn (Ch) dilution possibilitis basic colors. In this schm, th rcssiv Ch gn is idntifid by N. All offspring of homozygous dominant horss (ChCh) should show th Champagn dilution phnotypically. (from Cook t al., 2008) Bas color Modifid Rsult Furthr modifid Rsult any NN homozygous rcssiv for Ch. No chang in th hors s coat color from Ch gn. black aa NCh or ChCh dark tan brown points black AA, Aa +, Aa bay NCh or ChCh tan brown points chstnut NCh or ChCh gold 4 Alabama Cooprativ xtnsion Systm
Parl (Prl) dilution possibilitis basic coat colors. In this schm, th dominant Prl gn is idntifid by N, and th rcssiv is idntifid by Prl. (from Vtrinary Gntics Laboratory, 2011) Bas color Modifid Rsult Furthr modifid Rsult any NN or NPrl no chang black AA, Aa +, Aa bay NPrl and c cr psudo-prlino chstnut NPrl and c cr psudo-crmllo chstnut PrlPrl apricot body, man, and tail Gry(G), Roan (RN) and Whit (W) coat color modifications Bas color Modifid Rsult any GG, Gg gry any gg no chang any RNRN (probably roan lthal), RNrn any rn no chang any Ww whit any ww no chang Roan (RN) is anothr gn that lightns th bas coat color in horss by mixing whit hairs and colord hairs on th body. Th amount of whit hairs may vary btwn th summr and wintr coats, but ovrall, th hors rmains th sam color throughout its lif. Th had and lg colors of roan horss ar darkr than th body. Th roan color is a dominant trait, but thr is som controvrsy as to th possibility that th homozygous dominant condition is lthal in th arly mbryonic stat. Thr ar othr gns that contribut to an unvn roaning pattrn, typically around th flank, barrl, and top of th tail in horss. Th whit gn (W) modifis th normal coat so that th hors is whit from birth. Typically, this gn rsults in pink skin and dark ys. This is also a dominant trait, but rsarch indicats that th homozygous dominant (WW) is a lthal condition in arly mbryonic dvlopmnt. Thr is vidnc that two normal parnts can produc a whit foal, which indicats that som whit foals ar producd from a gn mutation. Th W gn is rar in all brds but can b found in th Tnnss Walking Hors, Miniaturs, and Amrican Albino brds and has occurrd in Thoroughbrds, Arabians, and Standardbrds. Tobiano (TO) and Ovro (O) spotting pattrns Bas color Modifid Rsult any TOTO, TOto tobiano any toto no chang any Oo ovro any oo no chang Th tobiano spotting color pattrn (TO) in which whit crosss th hors s back btwn th rs and tail is found in hors brds throughout th world. It is a dominant trait that can occur on any coat color. Th ovro spotting color pattrn (Oo) in which whit originats from th hors s blly and usually dos not cross th back btwn its rs and tail also is found in hors brds throughout th world. Bcaus an ovro offspring can rsult infrquntly from th mating of two solid-colord horss, it had bn assumd that th ovro color pattrn is causd by th homozygous rcssiv (oo). Howvr, transmission of th ovro spotting pattrn dos not follow a rcssiv pattrn and is now known to b inhritd as a dominant trait in which th homozygous dominant is a lthal condition in th arly mbryonic stag. This is not to b confusd th lthal whit foal syndrom, which also is associatd th ovro color pattrn. Foals th lthal whit foal syndrom ar almost always th rsult of two ovro parnts and ar typically ithr blu-yd whit foals or may hav only a fw colord spots around th muzzl, ars, or tail. Th lthal whit foal syndrom causs intstinal blockag du to missing portions of th digstiv tract or lack of nrv clls that control movmnt of th intstinal tract. Th sabino color pattrn is anothr spotting pattrn in which th hors has irrgular spotting on th lgs, blly, and fac and oftn has roaning in th colord portions of its coat. Sabino pattrns in diffrnt brds may b controlld by diffrnt gns. Basic Hors Gntics 5
Lopard complx (LP) appaloosa color pattrn Bas color Modifid Rsult any LPLP, LPlp appaloosa any lplp no chang Th appaloosa color pattrn is a combination of spotting and/or diffus roaning, which usually is fairly symmtrical on th hors s body. Appaloosa color pattrns ar found worldwid in various brds of horss and ponis. Until rcntly, th inhritanc of th pattrn was unknown, but gn mapping of th hors gnom has gratly incrasd th undrstanding of this color pattrn. Th gn rsponsibl for th appaloosa pattrn is trmd th lopard complx (LP). Th complx portion of th trm is to includ all typs of appaloosa pattrns that ar not lopard. It is blivd that LP functions as a dominant gn to caus th appaloosa coat pattrn, along othr appaloosa charactristics such as mottld skin, whit sclra, and vrtically stripd hoovs. To hav th color pattrn, th hors must hav an LP gn. Howvr, modifying gns may influnc th xprssion of th LP gn; thrfor, a hors that has minimal whit modifying gns may not xhibit th LP gn. To furthr confus th issu, many rsarchrs hav hypothsizd that th LP gn may act as an incomplt dominant gn. That is, a hors that inhrits an LP from both parnts (LPLP) will xhibit mor whit than th htrozygous hors (LPlp), which only rcivs th dominant form of th gn from on parnt. Horss that ar homozygous dominant ar fw-spot lopards, whil htrozygous horss ar lopard, blankt, varnish roan, snowflak, and frostd pattrns. In this publication, th gns that control color ar discussd individually, and prominnt brds xhibiting a particular color pattrn ar givn. Howvr, it is important to raliz that all horss hav a pair of gns for vry color gn discussd and many that ar not discussd. For xampl, a chstnut hors would hav th color gnotyp of dd CC gg ww zz toto oo lplp rnrn. Gntic tsts for som color gns xist. For xampl, a tst can b don to dtrmin whthr an animal is homozygous or htrozygous for th black gn. Th Quandary Quantitativ Traits Quantitativ traits oftn rsult from additiv gn action. That is, many gns affct th trait, and th ffct of ach gn is small. At th sam tim, th nvironmntal ffcts ar high for quantitativ traits. This combination tnds to blur th distinction btwn phnotypic classs. On hors may look bttr than its gntic makup would indicat bcaus it has bn in a good nvironmnt, whil anothr may look wors than its gntic makup would indicat bcaus it has bn in a bad nvironmnt. As a rsult, whn slcting brding animals, w may choos a hors a good phnotyp du to a suprior nvironmnt which, in rality, may or may not hav th gnotyp w dsir. Similarly, w may pass up a hors bcaus its phnotyp is undsirabl du to a poor nvironmnt, whil its gnotyp actually may b highly dsirabl for a spcific brding goal. In othr words, th bst prformrs may not always hav th most dsirabl gns, and nvironmntal factors can nhanc or mask gntic ffcts. Unfortunatly, many of our conomically important traits for horss ar quantitativ traits. Prformanc traits, such as racing spd, jumping ability, and cutting ability, ar all quantitativ traits, and valuation of prformanc usually has a combination of objctiv (such as spd and jump hight) and subjctiv (such as conformation and disposition) masurmnts. Animal brdrs hav a varity of formulas for prdicting gntic chang through brding. Ths formulas tak into account a varity of factors that influnc gntic chang. A simpl formula is as follows: yarly gntic chang = hritability slction diffrntial gnration intrval Som hors traits hav bn invstigatd to th xtnt that scintists hav stimatd th hritability of that trait. A highly hritabl trait gnrally mans that hors brdrs can mak mor accurat slction dcisions about that trait and can mor rapidly influnc th amount of gntic chang thir dcisions. Hritability stimats for various hors prformanc traits rang from 0.04 (cow sns) to 0.63 (r hight). Diffrnt populations of horss may show diffrnt hritability stimats for th sam trait bcaus thir gntic and nvironmntal backgrounds ar diffrnt. For xampl, if th population studid is an lit group of horss prforming at th top lvl of comptition, ths horss probably would b mor similar in nvironmnt and gntics than a group of horss prforming at th local or rgional lvl of th sam sport. To us an xampl from color gntics, if all horss in a population ar chstnut, thr is littl chanc of producing a black. Whnvr possibl, addi- 6 Alabama Cooprativ xtnsion Systm
tional information from rlativs, wightd proprly according to ach rlativ s rlationship to th animal bing slctd, will incras th slction accuracy rgardlss of th hritability for that trait. Th slction diffrntial is th diffrnc btwn th slctd individual and th population avrag. Brdrs who cannot prdict (many conomically important traits may not b masurabl until th animal maturs) or rcogniz suprior animals probably will not mak a lot of gntic chang or may nd up th wrong typ of chang for thir goals. Also, brdrs who hav only a limitd numbr of horss to utiliz in thir brding program ar at a disadvantag for this factor. Mor intns slction, through rtaining only th highst quality animals, allows brdrs to incras th man gntic lvl of thir hrd. Brdrs a limitd numbr of brding animals oftn do not want to intnsly cull animals bcaus it may rduc thir hrd siz to a numbr that is not conomically viabl. Howvr, producing animals of limitd usfulnss to th hors industry probably is not conomically viabl ithr. Th long gnration intrval of horss also rducs th rat of gntic chang in hors brding. Th gnration intrval is th avrag tim from th birth of on gnration to th birth of th nxt, and in horss th gnrally accptd gnration intrval is about 10 yars. Gntic chang can occur mor rapidly shortr gnration intrvals. Th long gnration intrval also mans that th avrag hors brdr has vry fw opportunitis to influnc gntic chang in horss in his or hr liftim. Poor slction dcisions or constantly changing brding goals will ngativly influnc th amount of gntic chang a brdr can raliz. Also, whn th long gnration intrval is combind poor rproductiv fficincy (only about 50 prcnt of mars brd produc a foal), this factor can gratly rtard gntic chang. Incrasing th rproductiv fficincy can dcras th gnration intrval and incras slction intnsity in a hrd, rsulting in mor rapid gntic chang. Finally, th vry natur of th hors industry can influnc th amount of gntic chang mad by individual brdrs. Historically, many dsirabl stallions wr accssibl to only a fw brdrs du to factors such as high cost and gographical location. This did not rtard th gntic chang of th brd, but it did slow th gntic chang ralizd by th avrag brdr. With th incrasd us of shippd smn in th hors industry, this problm is bginning to b allviatd in most brds. Through th us of artificial insmination, slction intnsity from th mal sid can b gratly improvd. valuating Individuals Brding stock slction is usually basd on a varity of individual prfrncs of th brdr. Som brdrs put mor wight on pdigr, som on prformanc of th hors or its rlativs, and som on apparanc of th hors. Ar som of ths considrations mor important than othrs? It dpnds on th trait in qustion. Highly hritabl traits (0.4 or highr hritability) rspond wll to slction procdurs basd on individual prformanc, assuming that nvironmntal ffcts ar minimizd. Individual prformanc is also a usful slction critrion for conomically important traits and whn gntic turnovr is fast. Traits low hritabilitis (lss than 0.2) rspond wll to slction basd on family background (pdigr, prformanc of rlativs) bcaus th individual s own prformanc is not an accurat assssmnt of th animal s gntic mrit. Pdigr also can b a usful slction tool for a young hors that has not had a chanc to prov itslf or for a hors that was injurd prior to proving itslf. Pdigr also may b a usful tool whn daling gntic abnormalitis, traits xprssd latr in lif, or traits xprssd by only on sx. Howvr, brdrs should raliz that th pdigr dcrass in valu in th slction procss as th individual animal gains prformanc and progny rcords. In addition, brdrs should rmmbr that th individual hors will not hav any gns that its parnts did not hav. So, whn utilizing th pdigr as a slction tool, brdrs should mphasiz th individual s parnts and grandparnts. Th gntic contribution of mor distant rlativs such as grat grandparnts is vry minor. Prformanc tsts, in which horss ar put into a common nvironmnt for a priod of tim bfor traits ar masurd and compard, and progny tsts, in which a stallion is brd to numbr of mars to valuat his offspring rlativ to thos of othr stallions for crtain traits, hav bn usd as slction tools. Othr livstock spcis commonly valuat potntial brding animals using this schm. Th prformanc tst commonly is usd as a slction tool for young stallions of warmblood brds. Basic Hors Gntics 7
Basic Brding Schms Brding schms of outcrossing and inbrding do hav gntic consquncs that may b ithr good or bad, dpnding on th goals of th brdr. Outcrossing is mating unrlatd familis in a brd. It tnds to incras htrozygosity, strngthn traits rlatd to fitnss (such as frtility), and covr up rcssiv gns. Crossbrding is a mor xtrm form of outcrossing in which horss from diffrnt brds ar matd. Crossbrding has th sam gntic ffcts as outbrding, plus it dcrass gntic purity. Crossbrding is common som typs and uss of horss. Many of our modrn light hors brds wr formd by crossbrding, and it is still usd today to introduc dsirabl gntics into hors populations. For xampl, Thoroughbrd and Arabian gns wr introducd into nativ hors populations to rfin th apparanc and incras th athltic ability of warmblood brds. Outcrossing is somtims usd for corrctiv mating for xampl, brding a splay-footd mar to a pigon-tod stallion. Ths two conditions potntially ar causd by two sparat gntic mchanisms, and it is difficult to prdict what typ of lg structur would rsult from th brding. A far bttr stratgy would b to brd th mar to a stallion th most corrct structur that fits into th brdr s conomic rstraints and othr slction critria. Inbrding is brding closly rlatd individuals (fathr-daughtr, brothr-sistr). Th gntic consquncs of inbrding ar to incras homozygosity of both dominant and rcssiv gns. Inbrding dos not crat undsirabl gns, but it dos incras th possibility that rcssiv gns will b paird and xprssd phnotypically. Inbrding also dcrass traits rlatd to fitnss, such as frtility, disas rsistanc, and longvity. Linbrding is a mild form of inbrding that brds horss rlatd to a common ancstor, and it has th sam gntic consquncs of inbrding. Inbrding and linbrding ar usd to st phnotypic typ in qualitativ (morphological) traits, to concntrat th influnc of an outstanding ancstor, and to form lins for futur outcrossing purposs. Nithr outcrossing nor inbrding is good or bad. Th us of ths brding schms dpnds on what is appropriat to rach th brding goals st by th brdr. Slction Mthods Aftr stting goals and xamining prformanc rcords, pdigrs, rlationships btwn animals, and tst rsults as appropriat, hors brdrs can us a varity of mthods of multipl trait slction. Bfor gtting startd, brdrs nd to raliz that as th numbr of traits slctd for incrass, th possibl amount of chang in a singl trait dcrass. On mthod is to slct for on trait xclusivly until th goal for that trait is rachd, thn start slcting for th nxt trait. This is trmd tandm slction and is probably th last ffctiv mthod of slction bcaus traits oftn ar corrlatd gntically. That is, on gn may affct mor than on trait. So, slcting for ach individual trait sparatly may altr, ithr positivly or ngativly, th gntic chang mad for th prvious trait(s). A scond slction procdur is to st minimum standards for svral traits, and if a hors dos not masur up to on of thos standards, it is culld. This mthod is calld an indpndnt culling lvl and is usd to som dgr by most brdrs and brd associations. Th problm this mthod is that a hors that is outstanding in on trait may b culld if it dos not mt th standards for anothr trait. This typ of slction is most usful if th brdr is daling only a small numbr of traits and a small prcntag of th offspring ar ndd to rplac th parnts. Nithr of ths rquirmnts is common to th hors population. A slction indx is a mor ffctiv mthod of trait slction than tandm slction or stting indpndnt culling lvls, but it is mor difficult to st up. Slction indics ar mathmatical formulas that utiliz information from traits of importanc and wight thos traits according to importanc. Information is obtaind from th animal itslf and from its rlativs to calculat a singl scor for an animal, and slction is basd on th scor. If multitrait valuations ar usd, th slction indx mthod simply wighs th traits by thir rlativ conomic valus and sums thm. Drawbacks to th slction indx ar that it is difficult to formulat, and it oftn is difficult to obtain th information ndd. Also, what may b important for on hors brdr and b wightd havily in his or hr indx would not ncssarily b a highly important trait to anothr brdr. Howvr, formulating a slction indx may b a usful xrcis bcaus assigning a rlativ conomic valu to ach trait allows th brdr to raliz which traits ar of tru conomic importanc to his or hr brding goals. 8 Alabama Cooprativ xtnsion Systm
Anothr mthod of slction that is commonly usd in othr livstock spcis and is bginning to b availabl for horss in urop is th xpctd progny diffrnc (PD). In this mthod, all availabl information about a trait from th individual and its pdigr, proprly wighd according to th rlationships to th animal bing valuatd, is usd to calculat th xpctd diffrnc in th offspring s prformanc for that trait. That is, an PD is th xpctd diffrnc in prformanc from futur offspring of a sir compard to th prformanc of th bas population. Th PD combins into on figur a masurmnt of th gntic potntial for a particular trait basd on th individual s prformanc and prformanc of rlatd animals. In othr spcis, PDs ar rportd as positiv or ngativ valus for ach trait of importanc. For xampl, a bf bull may hav an PD of -0.4 for birth wight (slightly blow th avrag of his prs), a waning wight PD of +38 (38 points abov th avrag of his prs), and a yarling wight PD of +82. So, if a particular brdr wr looking for a bull that sirs calvs of avrag birth wight that will grow rapidly aftr birth, this might b a bull to considr. Also, th procdur can b usd to rank th bull on traits that cannot b masurd dirctly from him, such as matrnal ability traits and carcass traits. Th PD scors com an accuracy scor attachd. Th mor offspring th sir has, th mor accurat th PD scor and th mor th brdr can rly on it to rmain rlativly unchangd additional data. In hors brding, th PD would b a good slction tool. Most brdrs hav too fw mars to cull thm intnsly. Instad, thy corrctly put mor mphasis on slction of th stallion. With th us of transportd smn, stallions ar abl to hav mor offspring in many diffrnt hrds. Proprly rcording th prformanc of ths offspring would allow for PD stimation so that th stallions could b compard mor objctivly by th brdr. Whthr th brding goal is to produc a palomino foal or to produc th nxt Olympic winnr, th sam basic gntic principls should guid th slction procss. Rfrncs for this publication and furthr rading matrials ar as follows: Bowling, A.T. 1996. Hors Gntics. CAB Intrnational, Nw York. Harrington, R.B. 1995. Animal Brding - An Introduction. Intrstat Publ. Danvill, IL Sponnbrg, D.P. 2003. quin Color Gntics. Iowa Stat Prss. IO. Vtrinary Gntics Laboratory. Univrsity of California, Davis. http://www.vgl.ucdavis.du/srvics/ coatcolorhors.php. Accssd Jun 30, 2011. Cook, D., Brooks S., Bllon, R., Baily,. 2008. Missns Mutation in xon 2 of SLC36A1 Rsponsibl for Champagn Dilution in Horss. PLoS Gntics 4(9):1000195 Cindy McCall, xtnsion Spcialist, Profssor Animal Scincs, Auburn Univrsity For mor information, call your county xtnsion offic. Look in your tlphon dirctory undr your county s nam to find th numbr. Publishd by th Alabama Cooprativ xtnsion Systm (Alabama A&M Univrsity and Auburn Univrsity), an qual opportunity ducator and mployr. Wb Only, Nw March 2012, ANR-1420 ANR-1420 2012 by th Alabama Cooprativ xtnsion Systm. All rights rsrvd. www.acs.du