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Summary We report on a complete genome scan for quantitative trait loci (QTL) affecting milk protein percentage (PP) in the Italian Holstein-Friesian cattle population, applying a selective DNA pooling strategy in a daughter design. Ten Holstein-Friesian sires were chosen, and for each sire, about 200 daughters, each from the high and low tails of estimated breeding value for PP, were used to construct milk DNA pools. Sires and pools were genotyped for 181 dinucleotide microsatellites covering all cattle autosomes. Sire marker allele frequencies in the pools were obtained by shadow correction of peak height in the electropherograms. After quality control, pool data from eight sires were used for all subsequent analyses. The QTL heterozygosity estimate was lower than that of similar studies in other cattle populations. Multiple marker mapping identified 19 QTL located on 14 chromosomes (BTA1, 2, 3, 4, 5, 6, 8, 9, 12, 14, 17, 20, 23 and 27). The sires were also genotyped for seven polymorphic sites in six candidate genes (ABCG2, SPP1, casein kappa, DGAT1, GHR and PRLR) located within QTL regions of BTA6, 14 and 20 found in this study. The results confirmed or excluded the involvement of some of the analysed markers as the causative polymorphic sites of the identified QTL. The QTL identified, combined with genotype data of these candidate genes, will help to identify other quantitative trait genes and clarify the complex QTL patterns observed for a few chromosomes. Overall, the results are consistent with the Italian Holstein population having been under long-term selection for high PP.

Fourteen Brazilian Gir sire families with 657 daughters were analyzed for quantitative trait loci (QTL) on chromosome 6 affecting lactose and total solids. Cows and sires were genotyped with 27 microsatellites with a mean spacing between markers of 4.9 cM. We used a 1% chromosome-wide threshold for QTL qualification. A QTL for lactose yield was found close to marker MNB66 in three families. A QTL for total solid yield was identified close to marker BMS2508 in three families. A QTL for lactose percentage, close to marker DIK1182, was identified in two families. A QTL for total solid percentage, close to marker MNB208, was identified in four families. These QTLs could be used for selection of animals in dairy production systems.

Chromosomal regions affecting multiple traits (multiple trait quantitative trait regions or MQR) in dairy cattle were detected using a method based on results from single trait analyses to detect quantitative trait loci (QTL). The covariance between contrasts for different traits in single trait regression analysis was computed. A chromosomal region was considered an MQR when the observed covariance between contrasts deviated from the expected covariance under the null hypothesis of no pleiotropy or close linkage. The expected covariance and the confidence interval for the expected covariance were determined by permutation of the data. Four categories of traits were analyzed: production (5 traits), udder conformation (6 traits), udder health (2 traits), and fertility (2 traits). The analysis of a granddaughter design involving 833 sons of 20 grandsires resulted in 59 MQR ( α = 0.01, chromosomewise). Fifteen MQR were found on Bos taurus autosome (BTA) 14. Four or more MQR were found on BTA 6, 13, 19, 22, 23, and 25. Eight MQR involving udder conformation and udder health and 4 MQR involving production traits and udder health were found. Five MQR were identified for combinations of fertility and udder conformation traits, and another 5 MQR were identified for combinations of fertility and production traits. For 22 MQR, the difference between the correlation attributable to the MQR and the overall genetic correlation was >0.60. Although the false discovery rate was relatively high (0.52), it was considered important to present these results to assess potential consequences of using these MQR for marker-assisted selection.