marker-assisted selection in wheat - ictsd

Chapter 13 – Marker-assisted selection in sheep and goats 2392001 and has been used extensively (over40 000 tests).The β-3 adrenergic receptor gene hasbeen sequenced (Forrest and Hickford,2000) and eight different alleles have beenfound. This allelic variation is significantlyassociated with increased risk of coldrelatedmortality of lambs.Meat traitsThe first causal mutation found for meattraits in sheep is the callipyge gene causingmuscular hypertrophy. The gene has beenmapped to chromosome 18 and the causativemutation has been identified. However,the trait is expressed in a rather complexmanner, termed polar over-dominance;only lambs that inherit the callipyge mutationfrom their father but not their motherdevelop the trait. Several interacting genesare involved and the complete molecularbasis of callipyge phenotypes has not yetbeen fully resolved (Freking et al., 2002;Cockett et al., 1996, 2005).The Carwell gene somewhat resemblesthe callipyge gene, as it has been mappedto the same genomic region (distal endof chromosome 18) and it also affectsmuscling (McLaren et al., 2001). However,the overall phenotypic effect is not exactlythe same in that the Carwell gene affectsonly the longissimus dorsi and unlike thecallipyge gene it has not been associatedwith a decreased tenderness if the meatis aged appropriately and neither doesit seem to be affected by the parent oforigin (Jopson et al., 2001). The functionalmutation of the Carwell gene, also knownas the rib-eye muscling (REM) gene, hasnot yet been found but close markers inlinkage disequilibrium with the putativegene are being developed in Australia,New Zealand and the United Kingdom. Acommercial gene test termed “LoinMax”was introduced towards the end of 2005 byOvita in New Zealand.A number of gene detection projectshave resulted in significant QTL for muscle,fat and other carcass traits, but not all ofthese have been published, confirmed orfine mapped. A number of studies havereported on QTL for meat traits in sheep(Broad et al., 2000; Walling et al., 2004;Johnson et al., 2005; McRae et al., 2005)and there are probably some unpublishedQTL being further developed. Some ofthese sheep QTL are based on related cattlegenes, e.g. the myostatin gene for doublemuscling (Grobet et al., 1997) and thethyroglobulin gene affecting intramuscularfat (Barendse et al., 2004).Wool traitsIn a recent paper, Purvis and Franklin (2005)reviewed QTL for wool production traitsand wool quality. Although wool traits canbe measured easily and have high heritability,these authors suggested that researchinto certain wool production genes was stilljustified, for example, to break antagonisticcorrelations (between fleece weight andfibre diameter) or to target specific woolquality traits important for the processingof the product.A few Mendelian (single locus)characteristics have been described forwool. There is a known mutation of thehalo hair gene (HH1) causing extremehairiness. This has been found in the NewZealand Romney breed and several lineshave been developed for the production of“carpet wool” using this specific mutation,.A recessive gene for hairlessness (hr) hasbeen described by Finocchiaro et al. (2003).Several QTL for wool traits have beenpublished (see Purvis and Franklin, 2005for an overview), but few of these havebeen confirmed. On the other hand, it is