Alpaca World Magazine Summer 2005 - Classical MileEnd Alpacas

Special Feature Genetic Improvementmany years to come we will not spendany time on describing the systems.Random mating can be very usefulin sire evaluation. If the sire’s mates arechosen for any particular reason then theresults may not be a true evaluation andmay give a false positive evaluation fromhaving a particularly good set of matesor a false negative result from havingparticularly poor mates.Assortive mating includes the conceptsof mating two like animals together whichis termed positive assortive mating ormating two animals that are dissimilarwhich is called negative assortivemating. Positive assortive mating couldbe illustrated by breeding tallest totallest, heaviest to heaviest, fi nest fl eeceto fi nest fl eece, etc. This increasesgenetic variation and tends to spread apopulation toward the two extremes ofthe characteristic for which it is practiced,and thus leads to less uniformity ofthe population. Since uniformity isusually desirable for most commercialanimal breeders, they normally considerthis increased phenotypic variation adrawback. Other breeders, attempting tocreate a very superior animal in somecharacteristic, for the purpose of theshow ring, are willing to accept the lossof uniformity for the chance of producingan extreme individual, which this matingsystem lends itself to. For a thoroughdiscussion of positive assortive matingsee a text on animal breeding 5 .Negative assortive mating , pairinganimals that are opposites or dissimilarfor a particular trait, is also sometimescalled corrective mating. This iscommonly seen in alpaca breedingtoday as breeders attempt to ‘correct’traits in a particular parent by selectinga mate that has a more desirablephenotype for that particular trait. It isnot unusual to hear a breeder say. ‘Iwant to improve her density’ or ‘I wantto put more fi ber coverage on her legs.’What they really mean is that theywant to breed this female with a malethat is likely to produce an offspringthat has a better value for the trait inquestion than the mother has. They willthen fi nd a male that has better valuesfor that trait, more fi ber density, morefi ber coverage on legs, etc. and assumethat breeding this sire to the dam theywant to ‘improve’ will generate a criathat will be better than the dam for thistrait. There are several assumptions andpossibly misconceptions involved here.First, we are assuming that the trait ishighly heritable, that is to say, the sire’sphenotype means that his breedingvalue for that trait is high enough to passit on to his offspring. We have alreadydiscussed phenotypic selection. Let usassume that for the trait in questionthe heritability is high enough to makephenotypic selection effective. Theoffspring then can be expected to besomewhere in value between the sireand the dam. So, if we have ‘improved’on the dam, haven’t we at the same time‘downgraded’ the sire? We never heara breeder say, ‘we want to decrease hisfl eece density,’ or ‘I want to decrease hisfi ber coverage’, but isn’t that in fact thesame thing? In fact, we have ‘bred to theaverage’. This is not a good strategy if youwant to maximise genetic rate of change,but it is a good strategy if you want tocreate uniformity of the herd. This can beadvantageous for some breeding goals,but one must realise that the herd isbecoming uniform around some geneticintermediate value.Inbreeding is one of the matingstrategies based on pedigree relationship.There is much difference of opinion aboutwhat constitutes inbreeding. Some wouldconstrain the term inbreeding to themating of full brother and sister or parentto offspring. Others would include the38 | Alpaca World Magazine Summer 2005