Alpaca World Magazine Summer 2005 - Classical MileEnd Alpacas

a quantum leap forward in the geneticimprovement of our breed.The key equation that determines theeffectiveness of our selection process,the rate of genetic change, states thatthe rate of genetic change is proportionalto selection accuracy, selection intensity,genetic variation and generation interval.The most basic method of estimatingbreeding value, and the most commonlyused in the North American alpacaindustry today, is called PhenotypicSelection. With phenotypic selectiononly the observable and measurablecharacteristics of the candidateconsidered for breeding, in other wordsthe phenotypic expression of the traitsbeing selected for, are used as selectioncriteria. For instance, if we want tobreed for high fl eece weight, and areconsidering if a particular female shouldbe selected for that breeding program,only the weight of her own fl eece wouldbe used to make the decision. Otherconsiderations, such as the geneticmerit (BV) of her sire and dam for fl eeceweight, and that of her siblings andoffspring, would be ignored. In fact, inthe case of most alpacas, those breedingvalues are not so much ignored asthey are unknown. The assumption inthis case is that a phenotype for fl eeceweight in alpacas is somehow related tobreeding value for fl eece weight. If that isnot true, then selecting the parent basedon his or her phenotype for this traitwould be worthless.The terminology used to describe justhow strong the relationship is between aphenotype value for a trait and breedingvalue for that trait is heritability. Ifheritability is high, selecting a parentbased on phenotype for a given traitis likely to get that phenotype in theoffspring. When heritability is low,observations and measurements ofthe value of an alpaca’s trait will notreliably predict what the offspring’s valuefor that trait will be. For example, inmost mammals fertility is not a highlyheritable trait. Therefore whether or not afemale conceives easily or often has littleto do with her breeding value for fertilityin her offspring.Heritability is important because itdetermines our accuracy when employingphenotypic selection. In turn it affectsthe rate of genetic return in our herd.See the key equation above. Phenotypicselection for a trait of low heritabilitywill give us poor selection accuracy andtherefore a slow rate of genetic change.Misconceptionsabout heritabilityOccasionally someone thinks that ifheritability is high, breeding value forthat trait is high also. This is not so. Ahigh heritability only tells us that therelationship between the breeding valuefor a trait and the phenotypic valuefor that trait is a strong one. Note thatheritability is a population measure, nota value associated with an individualanimal. If the heritability for a trait in agiven herd is high, the individual animalswill still each have their own individualphenotype, good or bad, for that trait.The high heritability just tells us thatthe parent is likely to pass that on tothe offspring. If, as an example, theheritability for fl eece weight is high, ananimal with a low fl eece weight is likelyto produce offspring with a low fl eeceweight; medium fl eece weight parent willtend to produce medium fl eece weightoffspring, etc. On the other hand, if theheritability is very low, a high fl eeceweight parent is just as likely to producea low fl eece weight child and a lowfl eece weight parent could produce ahigh fl eece weight offspring.Contrary to the assumptions of manybreeders, heritability is not an immutablecharacteristic of a trait. Heritability for atrait varies from population to populationand from environment to environment. Itis possible, at least to some degree, toincrease the heritability for a trait withina contemporary group. A contemporarygroup is a group of animals that are thesame sex, of similar age and have beenmanaged in the same fashion at thesame location. If we remember our basicequation for phenotype: P = G + EThen differences, or variations inphenotype are a result of changes ingenetics, environment, or both:∆P = ∆G + ∆EThe higher the genetic componentof this equation, the more geneticdifferences will affect phenotypicdifferences. In other words, theheritability will be high. This illustratestwo things. First, the more uniform wecan make the environment, in otherwords the smaller we can make thevariations in environment, ∆E in ourequation, then the more certain we canbe that variations in phenotype, ∆P,are the result of genetic differences,∆G. This is another way of sayingthat heritability will be higher; or thatthere is a stronger correlation betweenphenotypic variance and geneticvariance, which is breeding value. Wecan increase the heritability of a trait in apopulation by making the environmentalfactors as uniform as possible and byincreasing the precision and accuracyof our measurements. The second pointillustrated by this equation is that whatwe are looking at when we determineheritability is differences betweenanimals or variances. We are comparingvariances in phenotype with variancesin breeding values. Mathematicallydefi ned, ‘Heritability = the proportionof differences in performance for a traitthat are attributable to differences inbreeding value for the trait’ 2 . Sincevariance statistically is the square of thestandard deviation the mathematicalformula is then: h2 = σBV 2 /σP 2This is the most computationallyuseful formula for heritability. When atrait has low heritability, we cannot useour standard method of phenotypicselection to accurately determinebreeding value. We need other methodsof estimating BV in order to have anaccuracy of selection high enough toproduce an acceptable genetic rate ofchange. We will look at how this formulacan be used mathematically to makeseveral types of genetic predictions fromdifferent sources of information aboutthe individual, its siblings or its progeny,and to also calculate the accuracy ofthose predictions, in a future article.Selection IntensityThis factor of the key equation tellsus what percentage of a population isselected as parents. If our estimationof each animals BV is very accurate,but we allow every animal to reproduce,all we have done is shuffl e the existinggenes in the gene pool. There will beminimal if any genetic advancement andin general the overall genetic value ofthe herd cannot change. In other words,the overall gene frequencies of the herdwill not change. This truth stems fromthe Hardy-Weinberg equilibrium. Seethe author’s reference 3 or virtually anyother modern genetic text for a thoroughdiscussion of the Hardy-Weinberg Law.If the selection intensity is high; forinstance if only the top ten percent ofanimals, based on breeding values,are allowed to be parents, and if ouraccuracy of breeding value (selectionaccuracy) is high, then these intenselyselected parents should be far betterthan average genetically. As a result,their offspring should be equally superiorgenetically and the rate of geneticchange will be rapid.Financial considerations and the needto increase the number of alpacas inthe national herd make it very diffi cultfor North American alpaca breedersto exercise high selection intensity atthis time. It is the author’s impressionthat very few females, percentagewise, are culled from breeding today.A higher percentage of males areprobably kept from reproducing, butit seems that even with males there isonly moderate selection intensity today.This low selection intensity means thatestimating breeding values as accuratelyas possible is extremely important if weare to see genetic improvement in ournational herd. Differences in accuracyof selection, or EBV (estimated breedingvalues), can be very large. Selection ofeach alpaca, based only on phenotypicrecord, is not very accurate, particularlyif the heritability of the traits underconsideration is low. Greater accuracyin estimating breeding values can beachieved by using more information andmore sophisticated genetic predictiontechnology. These methods will bediscussed in a future article.MatingMating decisions, the third major factor,take place after selection has occurred.Selection is a herd wide process; matingis an individual process. Selectiondecisions determine which animals willreproduce; mating decisions determinewhich individual females will be bredto which individual males. There aremany different mating systems basedon existing stock and breeding goals.Certain mating strategies can beemployed for simply inherited traits;others are necessary for polygenic traits.Mating strategies for simply inheritedtraits are based on an understandingof basic Mendelian inheritance, anddo not require much discussion. If oneknows the number of loci involved, thenumber of alleles at each locus, andhow the alleles are expressed, onecan then determine the possible andprobable genotypes of the potentialparents and select mating combinationsthat will produce, or most likely producethe desired genotype in the offspring.Simple Punnett square diagrams can beused to plan matings that will producecertain gene combinations, homozygotes,heterozygotes, or epistatic combinations,for a simply inherited trait. Other matingstrategies that can be employed forsimply inherited traits are introgression,in which a specifi c allele existing in onepopulation can be brought into anotherpopulation in which it does not exist;and topcrossing, designed to createa purebred population or to convert apopulation from one breed to another.Since most, if not all, of the traits ofimportance to alpaca breeders seem tobe polygenic and quantitative in nature,it is more pertinent to spend our time onmating strategies for quantitative traits.‘A mating system can be defi ned as aset of rules for making mating decisions.As such, there is no limit to the numberof possible mating systems. There are,however, only a few general matingstrategies.’ 4These general mating strategiesfall into two types. One type is basedon animal performance, and can besubdivided into random mating andassortive mating, which is further dividedinto positive and negative assortivemating. The other type is based onpedigree relationships, and includesinbreeding and outbreeding.Random mating must be distinguishedfrom random selection; they are not thesame thing. Even very highly selectedanimals, once selected, can be allowedto mate with each other at random. Thereare numerous systems to assure that thematings are truly random. They are mostuseful in a large commercial operationwhere individual performance recordsare not available and the numbers ofanimals being dealt with are very large.Since this situation will not apply toalpaca breeders in North America forAlpaca World Magazine Summer 2005 | 37