Body size has long been considered a paradigm for quantitative inheritance due to its role in morphology, relative ease of measurement, and its genetic characteristics.  Body size is a quantitative trait which is affected by several genes of relatively small effect.  In order to identify where these genes are located within the genome, two strains of mice from Jackson Lab were crossed to produce an F₁ generation; the strain selected for large body size (LG/J) has an average mass of 37.4 g at 60 days of age and the strain selected for small body size (SM/J) has an average mass of 13.6 g at 60 days of age, making the difference in mass between the strains approximately 24 g.  The F₁ mice were crossed to produce F₂ mice, which were then randomly mated in a large population during each successive generation.  The necropsy traits for which quantitative trait loci (QTLs) are identified include: the masses of the heart, kidneys, liver, spleen, and fat pad, the total body weight, and the tail length.  After necropsy, DNA is extracted from the liver, a PCR is performed, and the PCR product is run on a 5% agarose gel.  The PCR product is used to determine the genotype of each mouse at each marker.  Due to recombination, there are differences in genotype between mice at different markers.  Between the markers, interval mapping is used to calculate what the most likely genotype is at each point along the chromosome.