A quantitative trait locus (QTL) is a polymorphic locus that contains alleles which differentially affect the expression of a continuously distributed phenotypic trait. Usually a QTL is a marker described by statistical association to quantitative variation in the particular phenotypic trait that is thought to be controlled by the cumulative action of alleles at multiple loci.
For a comprehensive review of QTL mapping techniques in the rat, see Rapp, J. (2000). Genetic Analysis of Inherited Hypertension in the Rat. Physiol. Rev., 90:135-172.
The annotation data file, human_QTL.gff, was downloaded from:
      
ftp://rgd.mcw.edu/pub/RGD_genome_annotations/human/archive/human_QTL.gff.020805
and processed to create two UCSC Genome Browser tables — rgdQtl and rgdQtlLink
— to enable this track.