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Quantitative trait loci (QTLs) with large effects on fatness and growth have been identified previously on pig chromosome 4 in an intercross between the European wild pig and Large White domestic pigs. Two F 2 sows, heterozygous for the actual chromosome region, were backcrossed to a Large White boar, and two backcross (BC 1 ) boars were in turn backcrossed to Large White/Landrace sows. One of the boars was heterozygous for an intact wild pig-derived QTL region, whereas the other carried a recombinant haplotype. A total of 85 BC 2 animals were produced. Phenotypical measurements included daily weight gain, ultrasonic measurements of fat depth at 70 and 90 kg and several carcass traits. QTL segregation was deduced using 15 markers previously assigned to chromosome 4. Highly significant QTL effects were observed on all fatness traits and on the length of the carcass. A small but significant effect on growth was also observed. The results confirm the presence of one or more QTLs on chromosome 4 affecting fatness and growth. There was a good agreement between the estimates of QTL effects in the F 2 and BC 2 generations. The results from the recombinant sire family allowed us to map the major QTL effect distal to the recombination breakpoint. We propose that this confirmed QTL with a major effect on fatness is designated FAT1.Keywords: abdominal visceral fat, growth, pig, quantitative trait loci, subcutaneous fat. IntroductionGenetic dissection of complex polygenic traits is a topical subject in humans, plants and animals (Lander & Schork, 1994;Paterson, 1995;Georges & Andersson, 1996). We have previously identified quantitative trait loci (QTLs) for growth and fatness traits in an intercross between two divergent populations of pigs, the European wild pig and the Large White domestic pig (Andersson et al., 1994;Knott et al., 1998). The study was the first genome-wide screening for QTLs in outbred animals. The QTLs with the largest effect were located on chromosome 4 and explained about 12% and 18% of the phenotypical variance in the F 2 generation for growth and fatness traits respectively. The wild pig alleles at these loci were associated with reduced growth and higher fat deposition, as expected because of the intense selection for lean growth in modern domestic pigs.It is essential that the presence and location of QTLs are confirmed before attempts are made towards their cloning or exploitation in animal breeding by marker-assisted selection. There are, in fact, several reasons why one may fail to confirm the QTL in a subsequent experiment: e.g. (i) the original observation may be a type I error (see Lander & Kruglyak, 1995); (ii) the QTL effect may depend on epistasis (Frankel & Schork, 1996); (iii) a large QTL effect may be caused by several linked QTLs each with a small effect, and the linkage may break up in subsequent generations; (iv) the estimated QTL effect may be seriously inflated when the power of QTL detection is low (Georges et al., 1995); (v) segregation at the QTL in the recipient population ma...

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Quantitative trait loci (QTLs) have been mapped in many studies of F2 populations derived from crosses between diverse lines. One approach to confirming these effects and improving the mapping resolution is genetic chromosome dissection through a backcrossing programme. Analysis by interval mapping of the data generated is likely to provide additional power and resolution compared with treating data marker by marker. However, interval mapping approaches for such a programme are not well developed, especially where the founder lines were outbred. We explore alternative approaches to analysis using, as an example, data from chromosome 4 in an intercross between wild boar and Large White pigs where QTLs have been previously identified. A least squares interval mapping procedure was used to study growth rate and carcass traits in a subsequent second backcross generation (BC2). This procedure requires the probability of inheriting a wild boar allele for each BC2 animal for locations throughout the chromosome. Two methods for obtaining these probabilities were compared: stochastic or deterministic. The two methods gave similar probabilities for inheriting wild boar alleles and, hence, gave very similar results from the QTL analysis. The deterministic approach has the advantage of being much faster to run but requires specialized software. A QTL for fatness and for growth were confirmed and, in addition, a QTL for piglet growth from weaning at 5 weeks up to 7 weeks of age and another for carcass length were detected.

The effect of the genotypes of five different blood protein loci (α1B-glycoprotein, A1BG; glucose phosphate isomerase, GPI; phosphogluconate dehydrogenase, PGD; postalbumin 1A, PO1A; transferrin, TF) on early body-weight traits was studied in one large population of Swedish Yorkshire breed pigs. A highly significant association was observed, between the transferrin genotypes and the piglet body weights, at 6 and 9 weeks of age. The TF BB type pigs were heavier than those of TF AB types at 3, 6, and 9 weeks of age, by 130, 340, and 370 g, respectively. In the light of previously published data, it was discussed that TF is an additional chromosome 13 marker that may affect early body weights in pigs. The other four loci studied, located on chromosomes 6 and 7, did not show any significant effect. ZUSAMMENFASSUNG: Zusammenhänge zwischen Transferrinlocus an Chromosom 13 und Ferkelgewichten Die Wirkung von fünf verschiedenen Blutproteinloci (αB-Glykoprotein, A1BG; Glukose Phosphat Isomerase, GPI; Phosphoglukonat Dehydrogenase, PGD; Postalbumin 1A, PO1A; Transferrin, TF) auf Ferkelwichte wurde bei Schwedischen Yorkshire Schweinen untersucht. Der Zusammenhang zwischen Transferrin Genotypen und 6 und 9 Wochen Gewichten war hochsignifikant, TF BB Ferkel waren bei 3, 6 und 9 Wochen Alter um 130, 340 und 370 g schwerer als TF AB Ferkel. In zusammenhang mit früheren Studien wird TF als ein weiterer Chromosom 13 Marker für Ferkelgewicht erörtert. Die anderen vier Loci an Chromosomen 6 bzw. 7 zeigten keine signifikante Wirkung.