Quantitative trait loci in dairy cattle

Detection and estimation of individual quantitative trait loci (QTL) is a prerequisite to application of marker-assisted selection (MAS), Study on dairy cattle has mainly focused on milk production traits. Most of the QTL mapping studies that are presently being carried out using the granddaughter design. When QTL-information is used for selection, genetic progress in other traits may be influenced as well, due to pleiotropic effects of QTL or closely linked QTL. QTL-information can be used in breeding schemes and marker-assisted selection to increase the rate of genetic improvement. Chromosomal regions showing effects on milk production seem to have effects on other traits. Multiple trait analyses should be used to determine whether pleiotropic gene effects play a role. If there are no negative effects on other economically important traits, MAS can be applied. Currently available methods offer excellent opportunities to identify more QTL. New initiatives, like sequencing the bovine genome, will further facilitate the identification of genomic variation, and its use. Due to the long generation interval and the high cost of each individual, all QTL analyses in dairy cattle have been based on the analyses of commercial populations using data on economic traits gleaned from existing data banks. In present review, we consider the experimental designs which have been applied for QTL detection in dairy cattle and then major identified QTL in dairy cattle will be discussed.