Bovine chromosomes 2 (BTA2) and 5 (BTA5) of purebred, half-sib progeny sired by five Japanese black bulls were genotyped using microsatellite DNA markers. The data were subjected to linkage analysis for the detection and mapping of segregating quantitative trait loci (QTL) influencing live weight, average daily gain and body measurements at weaning. Probability coefficients of inheriting allele 1 or 2 from the sire at specific chromosomal intervals were computed. The phenotypic data on progeny were regressed on these probability coefficients in a within-common-parent regression analysis. Fixed effects of sex, parity and season of birth as well as age as a covariate, were fitted in a linear model to the phenotypic data and subsequently analysed using QTL Express by generating an F-statistic through permutation tests at chromosome-wide significance thresholds over 10, 000 iterations at 1 cM intervals. Highly significant (P<0.01) segregating QTL for body measurements were detected on BTA2 for hip width (1 cM) and chest depth (8 cM) in Sire Family 1 and pin bone width (16 cM) in Sire Family 3. Other significant QTL (P<0.05) detected were withers height (3 cM), hip height (4 cM), body length (4 cM), shoulder width (6 cM), lumbar width (3 cM), thurl width (3 cM) and canon circumference (2 cM) in Sire Family 1, shoulder width (16 cM) in Sire Family 3 and thurl width (24 cM), pin bone width (19 cM), heart girth (26 cM) and abdominal width (69 cM) in Sire Family 4. Significant (P<0.05) QTL for live weight and average daily gain were detected on BTA2 for birth weight (5 cM) and weaning weight (3 cM) in Sire Family 1 and post-weaning average daily gain (68 cM) in Sire Family 4. BTA 5 contained QTL for birth weight, pin bone width and heart girth in Sire Family 3 that were only suggestive and not significant. Such localization of economically important QTL as demonstrated in this study, will expedite genetic improvement via marker-assisted selection, gene introgression and positional cloning in Japanese black cattle.
Key words: QTL mapping, Japanese Black cattle, BTA2, BTA5, body measurements, live weight, daily gain.
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