marker-assisted selection in wheat - ictsd

Chapter 12 – Marker-assisted selection in dairy cattle 217ferentially expressed in the mammary glandduring lactation, as compared with the liver.Furthermore, anti-sense SPP1 transgenicmice displayed abnormal mammary glanddifferentiation and milk secretion (Nemiret al., 2000).Schnabel et al. (2005) found concordancebased on four heterozygous and fourhomozygous sires for the United StatesHolstein population, as determined by agranddaughter design, while Cohen-Zinderet al. (2005) found concordance for threeheterozygous and 15 homozygous siresfrom both the United States and IsraeliHolstein populations. Cohen-Zinder et al.(2005) also analysed the site proposed bySchnabel et al. (2005), and found that thissite was hyper-variable in that at leastfour single nucleotide changes were foundwithin the 20 bp region centred on thepoly-A sequence. Eight of nine Israelisires analysed by the daughter design wereheterozygous for at least one of thesepolymorphisms.Many studies have found a QTLaffecting all five milk production traits andSCS near the middle of BTA 20. Blott et al.(2003) claimed that a mis-sense mutationin the bovine growth hormone receptorwas responsible for the QTL affectingmilk yield and composition on BTA 20,but did not find concordance for the bullsheterozygous for the QTL. Thus, this polymorphismmay be responsible for onlypart of the observed effect on BTA 20, ormay be a physiologically neutral mutationin LD with the QTN.For both the QTL on BTA 6 and 14,the polymorphisms analysed apparently donot account for the entire effect observed inthese chromosomal regions (Bennewitz etal., 2004a; Kuhn et al., 2004; Cohen-Zinderet al., 2005). The effect associated with themis-sense mutation in ABCG2 explains theentire effect observed on milk yield and fatand protein concentration, but does notexplain the effects associated with fat andprotein yield. It is likely that in the nearfuture additional QTN will be resolved.As noted, the meta-analysis (Khatkar et al.,2004) found significant effects on BTA 1, 3,9 and 10, in addition to the effects describedon BTA 6, 14 and 20.Methods and theory for MAS indairy cattleConsidering the long generation interval,the high value of each individual, the verylimited female fertility and the fact thatnearly all economic traits are expressedonly in females, it would seem that dairycattle should be a nearly ideal species forapplication of MAS. However, most theoreticalstudies have been rather pessimisticwith respect to the expected gains that canbe obtained by MAS. As noted by Weller(2001), MAS can potentially increase annualgenetic gain by increasing the accuracy ofevaluation, increasing the selection intensityand decreasing the generation interval.The following dairy cattle breedingschemes that incorporate MAS have beenproposed:• a standard PT system, with informationfrom genetic markers being used toincrease the accuracy of sire evaluationsin addition to phenotypic informationfrom daughter records (Meuwissen andvan Arendonk, 1992);• a MOET nucleus breeding scheme inwhich marker information is used toselect sires for service in the MOETpopulation, in addition to phenotypicinformation on half-sisters (Meuwissenand van Arendonk, 1992);• PT schemes in which information ongenetic markers is used to preselect youngsires for entrance into the PT (Kashi,