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

Milk production traits (milk production yield, milk fat yield, milk protein yield, milk fat percentage, milk protein percentage) are important quantitative traits for dairy cattle and have been focused by Dairy Herd Improvement (DHI). The goal of QTL mapping is to identify genes or QTN underlying these traits for us to gain a better understanding of their physiological and biochemical roles and for a more direct way of genetic improvement. Since the first QTL experiment on milk production was reported 14 years ago, many studies have reported mapping of QTL for milk production traits by linkage analysis. However, the mapping resolution achieved by linkage analysis is in general poor due to limited crossing-over events in pedigrees and marker density. Typically, the confidence intervals for many mapped QTL could span around 20～30 cM, which is too large for positional cloning of the underlying genes.The aim of the present study is to refine the position of previously detected quantitative trait loci (QTL) on bovine chromosome 6 affecting milk production traits in Chinese Holstein dairy cattle. A daughter design with 918 daughters from 8 elite sire famiilies and 14 markers spanning the previously identified QTL region were used in the analysis. All daughters were collected, from 746 inidividuals in linkage analysis to 918 individual in this analysis.The mean markers'density is nearly 1cM. We employed a combined linkage and linkage disequilibrium analysis (LDLA) approach with two options for calculating the IBD probabilities, one was based on haplotypes of all 14 markers (named Method 1) and the other based on haplotypes with sliding windows of 5 markers (named Method 2). For milk fat yield, the two methods revealed a highly significant QTL located within a 6.5cM interval (Method 1) and a 4.0cM interval (Method 2), respectively. For milk protein yield, a highly significant QTL was detected within a 3.0cM interval (Method 1) or a 2.5cM interval (Method 2). These results confirmed the findings of our previous study and other studies while greatly narrowed down the QTL positions. From the complete bovine genome sequence corresponding to this interval, the total of eighteen genes potentially relating to milk traits are involved, i.e., KLF3, TMEM156, WDR19, RPL9, LIAS, UGDH, UEB2K, RBM47, NSUN7, RPL7A, APBB2, UCHL1, TMEM33, SLC30A9, CCDC4, YIPF7, GABRA2. By the way, we extended the winQTLcart software to outbred animal population for analysis the milk production traits in Chinese Holstein population. Within family analysis confirmed the former result and reduce the confidence interval, while across family analysis didn't detect any significant QTL.