MAPPING QUANTITATIVE TRAIT LOCI FOR BODY-WEIGHT ON THE X-CHROMOSOME IN MICE - II - ANALYSIS OF CONGENIC BACKCROSSES

In a QTL mapping study with an F-2, population of mice, we have shown that one or more sex-linked factors account for a large part of the di vergence between mouse lines selected for high and low body weight. He re, we describe a study undertaken to map the putative X-linked quanti tative trait loci (QTLs) by backcrossing segments of chromosome from t he high line onto an inbred line derived from the low line, thereby re moving possible contributions from the autosomes and linked segments o f the X chromosome. Sublines containing a regional at the proximal end of the X chromosome were found to be associated with large difference s in body weight, and to account for almost all the difference between the lines. A Markov chain Monte Carlo based multipoint linkage analys is incorporating the available marker and phenotypic information from the backcross pedigree was used to map the QTL to a region of about 6 cM. There was no evidence for QTLs elsewhere on the chromosome. The es timated QTL effect is approximately 20 % of mean body weight in males and females at 10 weeks. From results obtained from this study and the accompanying F, analysis, we conclude the presence of a single factor for body weight localizing to about position (+/-SE) 26.4 +/- 1.2 cM on the X chromosome, which increases body weight by approximately 18% at 10 weeks. A strategy to positionally clone the QTL is discussed.