The genetic structure of sheep reflects their domestication and subsequent formation into discrete breeds. Understanding genetic structure is essential for achieving genetic improvement through genome-wide association studies, genomic selection and the dissection of quantitative traits. After identifying the first genome-wide set of SNP for sheep, we report on levels of genetic variability both within and between a diverse sample of ovine populations. Then, using cluster analysis and the partitioning of genetic variation, we demonstrate sheep are characterised by weak phylogeographic structure, overlapping genetic similarity and generally low differentiation which is consistent with their short evolutionary history. The degree of population substructure was, however, sufficient to cluster individuals based on geographic origin and known breed history. Specifically, African and Asian populations clustered separately from breeds of European origin sampled from Australia, New Zealand, Europe and North America. Furthermore, we demonstrate the presence of stratification within some, but not all, ovine breeds. The results emphasize that careful documentation of genetic structure will be an essential prerequisite when mapping the genetic basis of complex traits. Furthermore, the identification of a subset of SNP able to assign individuals into broad groupings demonstrates even a small panel of markers may be suitable for applications such as traceability.

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This review describes the pattern of intramuscular fat accretion in cattle and the potential for its manipulation during both the pasture (or backgrounding) and intensive grain-finishing phases of development. A growth curve for the development of marbling in British and Japanese Black type breeds is discussed with the conclusion that 3 phases of development exist: (i) a period of growth up to ~200 kg hot carcass weight where intramuscular fat does not increase; (ii) a period of linear development as carcass weight increases from 200 to 450�kg; and (iii) the attainment of mature body size (~500 kg carcass weight depending on genotype) at which intramuscular fat content appears to reachea maximum. Data are also presented to show that the intramuscular and other fat depots develop at similar rates indicating that intramuscular fat is not a late maturing depot. Pre-finishing growth checks reduce the initial intramuscular fat at the start of finishing and this is translated into lower levels at the end of finishing. It is argued that the greatest potential for the manipulation of intramuscular fat accretion during fattening is via an increase in the net energy of the ration. Increasing net energy can be achieved by increasing the cereal grain content of the diet (grain v. grass); by feeding processed cereal grain, which allows both maximal rumen fermentation and small intestinal digestion of starch, and by increasing the lipid content of the diet. In addition it is proposed that the substrate supply or hormonal milieu can also be optimised, along with the availability of net energy to maximise fat accretion. The role of lipolysis (fat turnover) as a regulator of fat accretion is also discussed.

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