Blank Yoela, Lewin Harris, Da, Yang, Wiggans, George

Somatic cell concentration is a quantitative trait with moderate heritability and is apparently affected by many different loci. Individual loci affecting quantitative traits (QTL) can be located via linkage to segregating genetic markers if the two loci are in linkage disequilibrium. In most early experiments, linkage disequilibrium was generated by crosses between inbred lines. For dairy cattle, this is not a viable option because of the long generation interval and expense of raising each animal. Thus, detection of QTL in dairy cattle must be based on analysis of records from existing populations. The goal of this study was to detect segregating QTL for milk production and secondary traits, including somatic cell score, in the US Holstein population via the granddaughter design with the aid of segregating DNA microsatellites. Five loci were found to have significant effects. Two of these loci affected both fat yield and percentage. The other three loci affected only one trait each: milk yield, somatic cell score, and herdlife. This is the first report of detection of QTL in dairy cattle for secondary traits. Mapping the bovine genome for QTL associated with somatic cell score will allow dairy breeders to accelerate genetic progress for disease resistance.