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Evaluation of approaches to detect quantitative trait loci for growth, carcass, and meat quality on swine chromosomes 2, 6, 13, and 18. I. Univariate outbred F₂ and sib-pair analyses¹

Department of Animal Sciences, University of Illinois at Urbana-Champaign 61801

	Abstract

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Results from univariate outbred F₂ interval mapping and sib-pair analyses of 12 growth and 28 carcass traits to identify QTL on SSC 2, 6, 13, and 18 were compared. Phenotypic and genetic data were recorded on a three-generation resource population including 832 F₂ pigs from a cross between three Berkshire sires and 18 Duroc dams. Thirty markers with an average spacing of approximately 16 cM were genotyped across the four chromosomes. The outbred F₂ mixed model included the effects of sex, birth month, and year, one-QTL additive, dominance and imprinting coefficients calculated every 1 cM using interval mapping, and a random family effect. The general sib-pair model used to describe the phenotypic differences between sib-pairs included the same systematic and random effects and a one-QTL additive coefficient calculated every 1 cM. The outbred F₂ analysis found significant evidence of QTL on SSC 2 associated with 105-d weight, backfat thicknesses, LM area, fat percent, shear force, juiciness, marbling, and tenderness. In addition, QTL were identified on SSC 6 relating to 42-d weight and LM area, and on SSC 18 for fat and moisture percents. In most instances, the outbred F₂ approach offered greater power to detect QTL; however, the sib-pair analysis offered greater power in several instances. The trait-specific superiority could be due to the relative advantage of each model within a trait data set. The two approaches provided complementary evidence for QTL segregating between the Berkshire and Duroc breeds used in the study that may be used to aid marker-assisted introgression and selection and candidate gene studies to improve swine growth and meat quality characteristics.

Key Words: Growth • Interval Mapping • Meat Quality • Outbred F₂ • Sib-Pair • Swine

	Introduction

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Genetic improvement results in increased pork production, while addressing consumers’ demand for consistent-quality, lean pork produced under humane conditions (Schinckel and Bennett, 1999). Detection of QTL enhances this genetic improvement particularly when the quality characteristics are measured on the carcass. Pioneering research by Andersson et al. (1994) reported QTL for growth and fatness on SSC 4 using 200 F₂ offspring from a cross between wild boar and Large White lines. Since then, numerous studies identifying QTL in swine using similar statistical approaches have been reported and have assisted in understanding the genetic architecture of economically important traits (Malek et al., 2001a,b; Sato et al., 2003; Thomsen et al., 2004).

The use of multiple statistical approaches to screen for QTL minimizes the probability of overlooking QTL and reporting false discoveries. One standard approach used to detect QTL in swine populations is the outbred F₂ model of Haley et al. (1994). This model relates the phenotypes of the F₂ offspring to QTL genetic effects. An alternative approach typically used in human populations is the sib-pair analysis that relates differences in the phenotype of offspring to differences in QTL genotypes (Haseman and Elston, 1972). Examination of the sib-pair approach to identify QTL and a comparison to the outbred F₂ analysis using field data have not been reported. The objectives of this study were to identify and characterize QTL influencing traits of economic importance in swine using the outbred F₂ and sib-pair analyses.

	Materials and Methods

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Population
Three purebred Berkshire sires were mated to 18 purebred Duroc dams to produce six F₁ males and 56 F₁ females. Between July 2000 and August 2001, most F₁ sows were mated to one of the six F₁ sires; however, a few F₁ sows had up to three litters from the same F₁ sire. A total of 832 (386 males and 446 female) F₂ pigs was recorded for phenotypic data. Approximately 96% of the F₂ pigs had information on 30 to 40 traits.

Phenotype Measurements
Twelve growth and 28 carcass traits were recorded following the guidelines of the National Pork Producers Council (NPPC, 1991). Pigs were weighed at birth and approximately every 21 d until d 240 or upon reaching a slaughter weight of approximately 109 kg. Diet, management, transportation, and holding procedures followed those described in detail by Hamilton et al. (2003). Briefly, pigs were placed in pens on the basis of weight and were housed in a controlled-environment facility at the University of Illinois. Pigs had ad libitum access to water and a corn-soybean meal–based finisher diet (as-fed, 15.8% CP, 0.67% lysine, 3,329 kcal of ME/kg) from a two-hole feeder. Pens of pigs were sent to slaughter, and on the afternoon before slaughter, pigs were weighed before loading and transported to a packing plant located approximately 280 km from the finishing facility. The pigs were held overnight at the plant in lairage (approximately 16 h before slaughter) without food but with access to water. All pigs were combined in one group during transportation and in lairage. Pigs were slaughtered using standard commercial procedures, including electrical stunning, exsanguination, and removal of hair. At slaughter, HCW (with head, leaf-fat and skin intact) and dressing percent were recorded. Postmortem pH was recorded on a sample of the LM at 45 min and 24 h, or ultimate pH, with the use of a pH probe. Remaining carcass measurements were recorded from the left side of each carcass at 24 h postmortem. The LM area was measured at three-quarters the distance across the LM at the 10th rib, and the area was determined by tracing with acetate paper. Hunter L*, a*, and b* scores, indicators of color, were measured on the cut surface of the LM using a Hunter LabScan spectrocolorimeter (model XE, Hunter Associates Laboratory, Inc., Reston, VA) set at D65 and 10° angle of reflection. Backfat thickness was measured with a ruler at the first, 10th, and last ribs, and last lumbar vertebra, and an average backfat thickness based on these measurements was calculated. External fat and connective tissue were removed from a 2.5-cm-thick portion of the LM, and the sample was homogenized and placed in a Whirl-Pak bag (Nasco, Modesto, CA), and subsequently frozen. The sample was used to determine percentage of moisture following the procedures described by Novakofski et al. (1989). The frozen samples were oven-dried at 110°C for 18 to 24 h, and fat was extracted with petroleum ether as described by Hamilton et al. (2003). Glycolytic potential (µmol/g of wet tissue) was measured in a 2.5-cm-thick chop of the LM by the procedure of Miller et al. (2000). Drip loss percent was determined as the weight lost from a 2.5-cm-thick chop of the LM stored in a Whirl-Pak (Nasco) bag in a 4°C cooler for 48 h. During the calculation of LM area, a trained professional evaluated color on a scale of 1 (light) to 5 (dark), firmness from 1 (soft) to 5 (firm), and marbling from 1 (lean) to 10 (fatty). Sensory traits scored by a trained panel of six independent testers using integer scales included juiciness (1 = dry to 15 = juicy), off flavor (1 = distasteful to 15 = none), and tenderness (1 = tough to 15 = tender). Cooking loss percent and shear force were evaluated using vacuum-packaged boneless chops from a 10-cm section of the LM that was stored at 4°C, aged at least 7 d, and frozen (–20°C) before evaluation. The samples were cooked on an open-hearth grill and weighed before and after cooking to measure cooking loss percent. Shear force was measured with a Universal Testing Machine (Instron, Canton, MA) using a Warner-Bratzler shear attachment as described by Hamilton et al. (2003). The number of F₂ pigs measured, the associated mean and SD, and narrow sense heritability estimates of 40 growth, carcass, and meat quality traits are given in Table 1. Estimates of narrow sense heritabilities for all traits correspond to a model including the fixed effects of sex, year and month of birth, and the random effects of sire, dam, and sire x dam that was evaluated using the REML software ASREML (Gilmour et al., 2002).

[1]

where fixed effects included sex (two levels, male and female), birth month and year (BYM; 13 levels = months from July 2000 to August 2001), and ß₁, ß₂, and ß₃ are the regression coefficients for the QTL additive (Add), dominance (Dom), and imprinting (Imp) effects computed every one centimorgan from the estimates of the QTL genotype probability (Knott and Haley, 1998; Seaton et al., 2002). Random effects of family (90 levels) and residual were assumed to be independently and normally distributed with a means of zero and variances of and , respectively. Live weight at slaughter was included as a covariate for all carcass and meat quality traits. Age at slaughter also was studied as a covariate; however, the results were mostly consistent with the model including weight, and only the latter results are presented. The model for birth and 21 d weight included parity (three levels = 1 to 3) and the covariate litter size (13 levels = 1 to 13). Analysis was performed using PROC GLM in SAS (SAS Inst., Inc., Cary, NC).

Evidence of QTL at every 1 cM was summarized in an F-test statistic. The chromosome locations with local F-test statistic maximums that surpassed the significance threshold were reported as putative QTL with the associated estimates of mode of action. A chromosomewise permutation test provided the distribution of the F-test under the null hypothesis of no QTL residing on the chromosome based on 1,000 permutations and the associated P-value at 0.05 and 0.01 thresholds (Churchill and Doerge, 1994). The genomewise F-thresholds were obtained from the average chromosomewise F-thresholds using a Bonferroni adjustment to account for the 18 chromosome pairs in the pig genome. The critical values of the F-test statistic were 6.33 and 7.86 for the 0.05 and 0.01 P-values, respectively. This Bonferroni adjustment is conservative for this study because only four chromosomes were studied.

Sib-Pair Analysis
Sib-pair analysis is based on the principle that phenotypic differences controlled by a QTL should decrease when siblings share more linked marker alleles identical by descent (IBD; Haseman and Elston, 1972). The general univariate sib-pair model at each putative position of the QTL was:

[2]

where z_i includes the squared phenotypic differences and sums between siblings in pair i, is the intercept, _qi is the overall probability that the sibs are IBD for a putative QTL based on full sib information, ß represents the change in z_i per IBD probability, and _i is the corresponding residual. The phenotypes were adjusted for all the systematic effects included in the outbred F₂ approach before the analysis was performed. The overall probability _qi is the average of the sire and dam IBD probabilities and is a function of the recombination fraction between the flanking marker(s) and the QTL (Knott and Haley, 1998; Visscher and Hopper, 2001). The residuals were assumed to be independently and normally distributed with a mean of zero and variance of ².

The sib-pair analysis was performed using QTL Express (Seaton et al., 2002). Evidence of QTL at every 1 cM was summarized in a t-test statistic and the chromosome locations, with local t-test statistic maximums that surpassed the significance threshold were reported as putative QTL (Knott and Haley, 1998). Thresholds of the outbred F₂ interval mapping analysis were applied to the sib-pair analysis because the outbred F₂ thresholds were typically more stringent than the sib-pair.

	Results and Discussion

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The narrow sense heritabilities ranged from 0.04 to 0.72 (Table 1) and were in agreement with estimates reviewed by Rothschild and Ruvinsky (1998). The majority of the traits had heritability estimates higher than 0.1, suggesting that QTL are segregating in this reference population and traits.

Chromosome 2
The outbred F₂ analysis identified putative QTL on SSC 2 associated with 10 traits (Table 3). A putative QTL with dominance mode of action at 37 cM was associated with 105-d weight. The heterozyous offspring had higher 105-d weight than the homozygous offspring (Table 4). The analysis also revealed evidence of QTL between 33 and 39 cM associated with 84-, 126-, and 147-d weight. The putative QTL associated with these weights displayed an additive mode of action (Table 4), where the alleles inherited from the Duroc line were associated with heavier pigs than the allele inherited from the Berkshire line.

The outbred F₂ analysis indicated a putative QTL with maternal imprinting effect located at 0 cM, near the telomeric region, affiliated with the last lumbar vertebra, 10th rib, and average backfat thicknesses (Table 3; Figure 1). In addition, evidence was found for a QTL with a maternal imprinting effect at 0 cM related to last-rib backfat thickness and LM area, a determining factor of lean tissue mass (Table 4; Figure 2). The results of the sib-pair analyses were generally consistent with the outbred F₂ analyses for fatness traits (Table 3). With this approach, QTL between 0 and 3 cM were associated with last lumbar vertebra, last rib, and average backfat thicknesses. A QTL at 7 cM corresponded to 10th-rib backfat thickness, and was located beyond the marker flanking the other backfat measurements; however, it was located within the highest outbred F₂ analysis peak. Sib-pair analyses identified QTL connected to last lumbar vertebra and average backfat thicknesses and provided evidence of a QTL connected to 10th-rib backfat thickness. Insulin-like growth factor-2, a paternally expressed gene located at the telomeric region of SSC 2, 0 cM, was shown to affect fatness traits (Jeon et al., 1999; Nezer et al., 1999; Van Laere et al., 2003).

View larger version (10K): [in this window] [in a new window]   Figure 1. The P-values (pval = log10[P-value]) for the QTL effect of average backfat thickness with chromosome locations in the outbred F2 analysis of SSC 2. Marker positions are indicated by an upward pointing arrows () on the abscissa. Horizontal lines indicate threshold values for genomewise 1% level (dashed line) and genomewise 5% level (solid line).

View larger version (12K): [in this window] [in a new window]   Figure 2. The P-values (pval = log10[P-value]) for the QTL effect of LM area with chromosome locations in the outbred F2 analysis of SSC 2. Marker positions are indicated by an upward pointing arrows () on the abscissa. Horizontal lines indicate threshold values for genomewise 1% level (dashed line) and genomewise 5% level (solid line).

Both analyses detected putative QTL for LM area at different locations on SSC 2. The sib-pair analysis detected a QTL at 53 cM, and there were two high peaks present in the LM area outbred F₂ analysis, where the largest peak occurred at 0 cM and the second highest peak at 53 cM. The 53 cM location was consistent with de Koning et al. (1999), who identified a QTL between 43 and 62 cM associated with backfat thickness, another measure of lean content.

The outbred F₂ approach offered evidence of QTL with additive modes of action at 30 and 40 cM associated with fat percent and tenderness, respectively (Table 3). The alleles inherited from the Berkshire line were connected to higher lipid content and tenderness scores than the alleles inherited from the Duroc line (Table 4). A QTL with an additive effect at 38 cM was related to shear force (Figure 3). The allele inherited from the Duroc line was associated with greater pressure required to shear the sample than the allele inherited from the Berkshire line. Malek et al. (2001b) identified a QTL with an additive effect at 143 cM related to tenderness. The Calpastatin gene has been mapped near the centromere of SSC 2; thus, it may be associated with the QTL related to tenderness and shear force because the QTL affiliated with the traits were relatively close in position and shared similar effects (Kuryl et al., 2003; Ciobanu et al., 2004). The QTL associated with fat percent was within the same marker interval as the QTL related to shear force, suggesting pleiotropic or linked QTL.

View larger version (12K): [in this window] [in a new window]   Figure 3. The P-values (pval = log10[P-value]) for the QTL effect of shear force with chromosome locations in the outbred F2 analysis of SSC 2. Marker positions are indicated by an upward pointing arrows () on the abscissa. Horizontal lines indicate threshold values for genomewise 1% level (dashed line) and genomewise 5% level (solid line).

A QTL affiliated with juiciness score was detected at 14 cM with an additive mode of action with the outbred F₂ analysis and 20 cM with sib-pair analysis (Table 3). The allele inherited from the Berkshire line was associated with juicier chops than the allele inherited from the Duroc line (Table 4). Evidence was found for a QTL with an additive effect at 18 cM associated with firmness score in the outbred F₂ analysis. The allele inherited from the Berkshire line was associated with greater firmness of a chop than the allele inherited from the Duroc line. In the sib-pair analysis, there was no statistically significant evidence for a QTL related to firmness, but the putative QTL was located at 20 cM. Both analyses detected a QTL with an additive effect at 20 cM related to drip loss percent. The allele inherited from the Duroc line was related to a greater quantity of water lost over a 48-h period than the allele inherited from the Berkshire allele (Table 4). The QTL for all three traits were within the same marker interval and had the same mode of action. In addition, drip loss percent influences the juiciness and tenderness of a cut of pork; thus, the traits may have been affiliated with the same or linked QTL (Baas, 2000). Dekkers et al. (2003) and Malek et al. (2001a, b) also identified QTL for juiciness, firmness, and drip loss percent on SSC 2.

Chromosome 6
Two QTL regions corresponding to several traits were identified on SSC 6 using the outbred F₂ analysis (Table 3). A QTL with an additive mode of action was identified at 110 cM related to 42-d weight (Figure 4). The allele inherited from the Duroc line was connected to heavier pigs at 42 d (Table 4). Evidence was found for a QTL with an additive effect at 111 cM associated with 105-d weight and with 84- and 126-d weight, although the latter traits had nonsignificant F-ratio values lower than 4.0. Likewise, the alleles inherited from the Duroc line were connected to heavier pigs than the allele inherited from the Berkshire line. The same QTL may be affiliated with 42- to 126-d weight because the QTL for these weights shared the same position and mode of action. Our findings are consistent with Sato et al. (2003), who identified QTL at 102 cM connected to 30-d weight, and Bidanel et al. (2001) who identified QTL at 132 and 134 cM for to 10- and 13-wk weight, respectively.

View larger version (12K): [in this window] [in a new window]   Figure 4. The P-values (pval = log10[P-value]) for the QTL effect of 42 d weight with chromosome locations in the outbred F2 analysis of SSC 6. Marker positions are indicated by an upward pointing arrows () on the abscissa. Horizontal lines indicate threshold values for genomewise 1% level (dashed line) and genomewise 5% level (solid line).

A QTL with an additive mode of action at 95 cM was related to LM area. The allele inherited by the Duroc line was associated with a higher amount of muscling than the allele inherited from the Berkshire line. Sib-pair analysis identified a QTL at 124 cM for LM area that was within the peak identifying the QTL in the outbred F₂ analysis.

Evidence was found in the outbred F₂ analysis for QTL with maternal imprinting effects at 72 and 71 cM associated with last lumbar vertebra and average backfat thicknesses, respectively. Evidence was found for a QTL with a paternal expression at 62 cM connected to last-rib backfat thickness that was located outside the marker interval flanking the putative QTL influencing the other backfat measurements. Sib-pair results for backfat measurements were not consistent with those of outbred F₂ analyses. Bidanel et al. (2001) identified QTL at 61 and 63 cM affiliated with average backfat thickness at 13 and 17 wk of age, respectively. Gerbens et al. (2001), Grindfleck et al. (2001), and Ovilo et al. (2002) postulated that heart fatty acid binding protein (H-FABP) located at 84 cM influences backfat thickness. Thus, the QTL associated with one or more backfat measurements in our study may be H-FABP. Evidence was found for QTL with additive effects at 71 cM connected to carcass weight and yield or dressing percent. The alleles inherited from the Duroc line were related with higher carcass weight and yield than the alleles inherited from the Berkshire line. The mode of action differed between the QTL influencing backfat and carcass measurements, suggesting multiple QTL between 71 and 72 cM related to different sets of traits.

The outbred F₂ analysis provided evidence for a QTL with an additive mode of action at 23 cM related to moisture percent (Table 3). The allele inherited from the Duroc line was affiliated with higher amounts of moisture than the Berkshire line (Table 4). Evidence was found for a QTL with an additive effect at 19 cM connected to fat percent. The allele inherited from the Berkshire line was connected to higher lipid content than the allele inherited from the Duroc line. The same QTL may be affiliated with the traits because they are negatively correlated, located within the same marker interval, and share the same mode of action. This negative relationship probably occurs because as fat percent increases, fat cells fill with lipid, and cytoplasm (moisture) is diluted (Kiser et al., 1995). Evidence was found for a QTL with an additive effect at 128 cM connected with carcass length. The allele inherited from the Berkshire was associated with longer carcasses than the allele inherited from the Berkshire line. Malek et al. (2001) and Sato et al. (2003) also identified QTL related to carcass length at 141 and 138 cM, respectively.

Sib-pair analyses did not identify QTL associated with moisture and fat percents, and the results were not consistent with the outbred F₂ analyses (Table 3). Evidence was found for a QTL at 156 cM related to carcass length that differs from the outbred F₂ location for carcass length, 128 cM. The position of the QTL connected to carcass length identified by the outbred F₂ approach was located within the peak that identified the QTL in the sib-pair analysis. Sib-pair analysis revealed a QTL at 57 cM related to tenderness that was not identified with the outbred F₂ approach. Gerbens et al. (1997) mapped the H-FABP candidate gene with potential influence on tenderness to SSC 6. Therefore, identification of a QTL connected to tenderness may be the result of H-FABP.

Chromosome 13
The outbred F₂ analysis found evidence for QTL related to several traits on SSC 13 but the sib-pair approach did not identify any significant regions on SSC 13 (Table 5). Evidence was found for QTL at 80 and 77 cM affiliated with Hunter a* and b* color, respectively. The F-ratio value for Hunter b* color was lower than 4.0 and was not included in Table 5. The alleles inherited from the Duroc line were associated with darker shades of red and yellow color in pork than the allele inherited from the Berkshire line (Table 6). de Koning et al. (1999) also identified QTL at 95 and 113 cM corresponding with Hunter a* and b*, respectively. Strong evidence was found for a QTL with an additive effect at 88 cM associated with fat percent. The allele inherited from the Duroc line was connected to higher lipid content than the allele inherited from the Berkshire line. Strong evidence also was found for a QTL with an additive effect at 86 cM related to moisture percent. The allele inherited from the Berkshire line was related to higher amounts of moisture than the allele inherited from the Duroc line. The traits may have been influenced by the same or linked QTL because the detected QTL were negatively correlated, located within the same marker interval, and shared the same mode of action.

Chromosome 18
Two QTL regions were identified on SSC 18 related to several carcass and meat quality traits in the outbred F₂ analysis, but no QTL were detected with the sib-pair analysis. A QTL with an additive mode of action at 69 cM was connected to fat percent (Table 5). The allele inherited from the Berkshire line was related to higher lipid content than the allele inherited from the Duroc line (Table 6). Another QTL with an additive effect at 69 cM was related to moisture percent. The allele inherited from the Duroc line was connected to higher amounts of moisture than the allele from the Berkshire line (Table 6). The traits may have been affiliated with the same or linked QTL, because fat percent is negatively correlated with moisture percent, and the QTL share the same position and mode of action. The t-test maxima for fat and moisture percents (70 and 67 cM, respectively) obtained with the sib-pair analysis (Table 5), although nonsignificant, were at locations consistent with the results (69 cM) from the outbred F₂ analysis.

The outbred F₂ analysis detected a QTL between 40 and 63 cM associated with backfat measurements (Table 5). Putative QTL with additive effects at 40 and 43 cM were related to last lumbar vertebra and last-rib backfat thicknesses, respectively (Table 6). The QTL alleles inherited from the Berkshire line were associated with higher backfat thickness than alleles inherited from the Duroc line. Sun et al. (1997) reported that growth hormone releasing hormone receptor, located between 43 and 45 cM, might influence growth and carcass characteristics.

In the outbred F₂ analysis putative QTL with additive effects at 65 and 63 cM were associated with 10th-rib and average backfat thicknesses, respectively (Table 5). The alleles inherited from the Berkshire line were associated with higher backfat thickness than the allele inherited from the Duroc line (Table 6). The same or linked QTL may have been connected to the two backfat measurements because the QTL are located within the same marker interval and share the same mode of action. The QTL for 10th rib and average backfat thicknesses were located in a different marker interval than the QTL for last lumbar and last-rib backfat thicknesses; therefore, the two groups of backfat measurements might be affiliated with more than one QTL. The nonsignificant putative QTL identified for tenth rib and average backfat thicknesses by sib-pair analysis were located within the same marker interval as the QTL identified by outbred F₂ analysis (Table 5). Demeure et al. (2004) reported that the gene encoding for an isoform of AMP-activated protein kinase chain, PRKAG2, believed to affect backfat thickness was mapped to SSC 18. The PRKAG2 gene has been mapped to a location with no statistically significant evidence for QTL; however, it is possible that the candidate gene may influence backfat measurements in other populations (Bidanel and Rothschild, 2002).

Comparison of Approaches
The outbred F₂ approach of Haley et al. (1994) generally provided slightly more significant putative QTL than the sib-pair analysis of Visscher and Hopper (2001). Therefore, the outbred F₂ analysis seems to have greater statistical power than the sib-pair analysis for the markers and traits analyzed in this study. The difference occurs because the outbred F₂ and sib-pair approaches model different phenotypic descriptors (individual phenotypes and sib phenotypic information, respectively) and use different QTL indicators. The outbred F₂ approach incorporates additive, dominance, and imprinting effects, whereas the sib-pair approach uses IBD probabilities of QTL with additive effect. Hence, the outbred F₂ analysis may detect QTL with a relatively weak additive mode of action, which the sib-pair analysis may fail to detect. Furthermore, power of detection is dependent on family size because as family size increases, the potential quantity of information available on markers IBD increases (Gotz and Ollivier, 1992).

	Implications

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Quantitative trait loci influencing growth, carcass, and meat quality traits segregating in a cross between two commercial swine breeds were detected using different statistical approaches. This study demonstrated that sib-pair analysis supplements the information from outbred F₂ quantitative trait loci mapping studies in swine populations. The outbred F₂ and sib-pair analyses resulted in consistent identification and characterization of quantitative trait loci influencing many traits of economic importance, including backfat, longissimus muscle area, fat and moisture percents, juiciness, tenderness, shear force, pork color, cooking loss, and weight at different ages in swine. The consistency of the quantitative trait loci detected by both approaches on the same data set may improve the ability to decrease false positive results. The abundance of information generated through combining these two approaches may be used to assist in marker-assisted introgression and selection, fine mapping, and candidate gene approaches.

	Footnotes

 
¹ Acknowledgments: This study was funded by the USDA-IFAS, Grant #00-52100-9610 and C-FAR, Grant # 01I-121-1-UIUC.

² Correspondence: 1207 W. Gregory Dr. (phone: 217-333-8810; fax: 217-333-8286; e-mail: rodrgzzs{at}uiuc.edu).

Received for publication September 30, 2004. Accepted for publication April 12, 2005.

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Top Abstract Introduction Materials and Methods Results and Discussion Implications Literature Cited

 


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