Mapping QTLs On BTA6 Affecting Milk Production Traits In A Chinese Holstein Population

A Holstein population with 2650 individuals was genotyped by 14 known microsatellite markers to map the QTL affecting milk production traits on BTA6 (Bos Taurus Autosome 6). Microsatellites were amplified by PCR using fluorescent-labeled primers, and genotyped on the ABI 377 DNA sequencer with GeneScan v. 3.1 and Genotyper v. 2.5 used. After deleting the animals whose marker genotype did not match with the paternal genotype for more than 3 marker loci and the sire families with less than 30 daughters, 2,356 daughters of 26 sire families were used for the analysis, with a range of 35 to 195 daughters per sire. 11,055 phenotype records with at least 5 testing days were included in MT-DFRMEL analyses by a repeatability animal model to calculate the EBVs of the 305-day milk production traits. With these, a first largest scale QTL mpping was then employed.Two different approaches, linear regression approach and variance component approach, were implemented with QTL Express and MQRMEL respectively. 5 milk production traits were analyzed, which were milk yield, fat yield, protein yield, and fat percentage and protein percentage. With a one-QTL model, the QTL Express revealed a QTL near BMS470 with effects on milk yield, fat yield, protein yield, and fat percentage, and another QTL near BMS2460 for protein percentage, where the QTL for fat yield was at 5% chromosome-wide significance level. Eight different families with significant QTL effect on at least one trait were identified (P<=0.05). In the analyses across the significant families for corresponding traits, QTLs for all five traits were revealed at 1% or 5% experiment-wide significance level. The test statistic values near BMS1242 were also rather high as at BMS470 when analyzed for the fat yield across the significant families.Estimation by MQREML was carried out for the significant families detected by QTL Expresss, as there was a limitation for calculation in the second method. It was found that the QTLs affecting the 3 yield traits were all significant at P<=0.02, but the QTLs affecting the 2 percentage traits were not confirmed. For the milk yield and protein yield, the QTLs were fixed near BMS470. But the QTL affecting fat yield was positioned near BMS1242.With a two-QTL model, both approaches provided strong evidence for the existence of two QTLs for the three yield traits (P<0.05). For the two percentage traits, it was estimated there was only one QTL or no QTL. In this study, two different MARC linkage maps about BTA6 were based, with the results discriminating mainly at the positions near BMS1242 and BM143, where changes of the test statistic values resulted in the the lengths of the confidence intervals of the QTLs. When MARC97 map was based and MQREML used, very small confidence intervals were revealed for the QTLs affecting the 3 yield traits, only 4-5 cM. It would give a very good basis for finding the actual genes within these intervals.