Refined mapping of quantitative trait loci for somatic cell score on BTA02 in the German Holstein

The aim of this study was to more precisely map a previously reported quantitative trait locus (QTL) affecting somatic cell score on Bos taurus autosome 2 by increasing the number of markers fourfold, analysing more families and exploiting within-population linkage disequilibrium (LD). A granddaughter design of 10 German Holstein grandsire families with 1121 progeny tested sons was used. Twenty-six markers with an average marker spacing of 3.14 cM were genotyped along 81.6 cM. Linkage analysis (LA) was performed using variance-component methodology. The incorporation of LD was first done using variance-component methods followed by regression on marker alleles. LA revealed genome-wide significance (LOD > 3) at 15 contiguous marker-intervals, with the maximum test-statistic between DIK2862 and BMS778 and a 1-lod drop-off interval of 38 cM. While the variance-component methods could not detect any LD, two individual markers with a significant effect (ILSTS098, p < 0.05; BMS778, p < 0.01) were found by regression analysis. Compared with previous results QTL-localisation was substantially narrowed; further fine-mapping should focus on the close vicinity of BMS778.